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AlGaAs-GaAs lasers grown by metalorganic chemical vapor deposition — A review
Journal of Crystal Growth 55 (1981) 213 222 North-Holland Publishing Company
213
AIGaAs GaAs LASERS GROWN BY METALORGANIC CHEMICAL VAPOR DEPOSITION R.D. DIJPUIS
—
A REVIEW
*
Bell Laboratories, Murray Hill, New Jersey 07974, USA
High quality double-heterostructure (DH) lasers having excellent device characteristics have been grown by metalorganic chemical vapor deposition (MOCVD), as reported by a number oflaboratories. In addition to devices employing conventional planar DH laser geometries, high-performance lasers having unique device structures have also been demonstrated. This paper will review recent world-wide progress in the application of MOCVD to the growth of AIGaAs GaAs laser structures.
1. Introduction The metalorganic chemical vapor deposition process is an open-tube vapor-phase thin-film crystal growth technology that employs metal alkyls (or organometallics e.g., trimethylgallium, (CH3)3Ga), and hydrides (e.g., arsine, AsH3), as sources of the elements of which the thin film is to be made. The metalorganic chemical vapor deposition (MOCVD) process was first demonstrated by Manasevit in 1968 [1]. Much of the early research on the growth of III V compound semiconductor films by this vapor-phase materials technology was concentrated on the growth of heteroepitaxial thin films of various III V materials upon insulating oxide substrates, e.g., A1203 and BeO [2]. More recently, however, the primary interest in MOCVD has been for the growth of semiconductor thin films on closely-lattice-matched semiconductor substrates. for the fabrication of III V semiconductor heterojunction devices such as lasers, detectors, and field-effect transistors (FET’s). This paper wifi review the development of the MOCVD materials technology for one of these specific device applications the MOCVD growth of Al~Gai~As GaAs heterojunction structures for lasers. In addition to conventional broad-area and stripe-geometry double-heterostructure (DH) lasers, many novel —
*
Work was done while Anaheim, California, USA.
at Rockwell International,
0022-0248/81/0000 0000/$02.50 © 1981 North-Holland
laser structures have been grown by MOCVD, providing improved and unique device performance characteristics.
2. MOCVD materials growth for lasers The majority of the MOCVD research for A1GaAs DH lasers has employed trimethylgallium (TMGa) and trimethylaluminum (TMA1) as sources of the column III metals and arsine (AsH3) as the source of the column V element As. The doping of the pand n-type AlGaAs and GaAs epitaxial layers required for an injection laser structure is typically accomplished by employing diethylzinc (DEZn) and H2Se, respectively [3]. The epitaxial growth on an appropriate substrate is accomplished in an open-tube reactor by the pyrolysis of vapor-phase mixtures of these compounds at 650 750°C in H2 at (or slightly below) atmospheric pressure. For Al~Ga1~Asalloy films, the alloy composition is controlled by the relative initial partial pressures of TMGa and TMAI established in the reactor during growth. Similarly, the n-type Se and ptype Zn doping levels in the epitaxial films are controlled by the initial partial pressures of the H2 Se and DEZn sources. A schematic diagram of the atmosphericpressure MOCVD reactor that Dupuis and Dapkus have employed for the growth of DH lasers is shown in fig. 1 [3,4]. Since the metalorganics TMGa, TMA1, and DEZn are liquids near room tem-
214
R.D. Dupuis/AIGaAs GaAs lasers grown by MOCVD MASS FLOW CONTROLLERS
2
QUARTZ REACTOR ~WER
VACUUM
Fig. 1.
AsH~
H
EXHAUST
p
.L
I OEZn
~
~-1~J
technology make it wellsuited for the growth of uniform large-area submicron epitaxial thin films required for the cost-effective production of DH 2 in areaGaAs have beenlaser grown by lasers. In fact, Al~Ga1~As DH wafers as larger than 11 cm ::on:ntio~al DH lasers grown by MOCVD
~~~FIED
SUSCEPTOR
These characteristics of the MOCVD materials
DEWAR FLASK
3.1. Pulsed broad-area lasers
Schematic diagram of MOCVD reactor. The
perature with relatively high vapor pressures, they can be readily transported into the reactor by bubbling a H2 carrier gas through the liquid. The hydrides AsH3 and H2 Se are gases at room temperature and are usually supplied as mixtures in H2. The graphite susceptor is heated by an RF induction coil that is external to the quartz reactor chamber. The susceptor is usually rotated during growth. While most MOCVD lasers have been grown by this atmospheric-pressure MOCVD process, Hersee et al. have reported the growth of AlGaAs DH lasers by low-pressure MOCVD (LPMOCVD) using a horizontal reactor tube [5]. During growth, a roughing pump is used to reduce the pressure in the reactor to —~l00Torr. Layer thickness control and uniformity are excellent for MOCVD epitaxial layers. This is important for the reproducible growth of high-yield DH laser wafers. Even using deposition conditions corresponding to growth rates ~‘3000 A/mm, layers as thin as ‘-~50A can be reproducibly grown. While the uniformity of such a thin layer is not easily measured, SEM measurements of the thickness of ~1 pm thick MOCVD epitaxial Al~Gai_~As layers have shown that the thickness variation is less than ±1.5%over greater than 80% of the length of a 4.3 cm long bar cleaved from an MOCVD DH laser wafer [6] In addition to layer thickness control and uniformity, the MOCVD process offers excellent control and uniformity of the growth rate, doping density, and alloy composition of A1GaAs layers [7], as well as low interface recombination velocities at A1GaAs GaAs heterojunctions [8]. -
conventional Al~Ga
1 ~As GaAs doubleheterostructure laser typically employs a three-layer symmetrical dielectric waveguide consisting of a central GaAs “active region” and two Al~Ga1 ~As “passive” cladding or confining layers. Such structures are readily grown by MOCVD. In practive, five epitaxial layers are grown. The first layer is typically a 0.3 1 .0 pm GaAs: Se n-type buffer layer. Then a 1.0 2.0 pm Al~Gai_~As : Se n-type confining layer, a 0.01 0.3 pm undoped GaAs active layer. a 1.0 2.0 pm Al~Ga1~As : Zn p-type confining layer, and an 0.3 1.0 pm GaAs: Zn p-type “cap” layer are sequentially deposited in a single MOCVD growth cycle [3]. Broad-area injection lasers with cleaved facets and sawed sides are then fabricated from the wafer. Typical dimensions are 200 600 pm between Fabry Perot laser facets and 200 pm between sawed sides. These devices are then tested under pulsed current excitation employing ‘~200ns pulses at repetition rate ~-‘l kHz. The spectral output is measured as a function of peak drive current and the current required for the onset of laser operation is determined. The dependence of the threshold current density, ~th, of broad-area MOCVD DH lasers upon the active layer thickness, d, and the alloy composition, x, of the Al~Gai ~As confining layers is shown in fig. 2. The closed data points give the lowest values of ~th measured for broad-area MOCVD lasers [6]. For comparison, the lowest reported values measured for comparable Al~Ga1~As DH lasers grown by liquidphase epitaxy (LPE) are also shown (open data points) [9]. The lines of fig. 2 show the dependence of ~th upon d and x determined by an approximate calculation [6]. As shown in fig. 2, MOCVD DH
RD. Dupuis / AIGaAs GaAs lasers grown by MOCVD ~~.1
3000
MO-CVD A x = 0.27 . x =0.32 • x —0.50
E
~ 2500 ~
~
(A
2000
-x
= =
ILL
~ ~
0.20 0.27
0
0
L PE — 0 25 ~x =045~ ~x = 0.50 = 0.55 ~x = 0.68
215 2
LPE
36 380 X 200 0:25-0.35 :500x 100 MO - CVD V,~ x = 0.27 -0.30 .—~5O0X 200 MBE • x =0.30 ‘375 x 200 0
4.0 E
~ILI 1500 W//X=0.50 W/ ~ ~ /
~
x
DIODEAREA(~m)
0
X=
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A
0
2~
3.0
-
(AVERAGED ~th) CURVE a 2~m)
~ =
500 I
2.0 I
~-*=
/
5160 A/cm
~
GaAs 00500 pulsed ACTIVE broad-area 1000 LAYER 1500 THICKNESS, DH lasers 2000 d (A) grown 3000 ?500 by MOCVD Fig. Dependence and x for Al~Gai xAs (open2. points) and of for~ththeupon bestd comparable lasers grown by LPE (closed points).
/
1.0- ~% ~ (—~=
I
0
broad-area lasers exhibit values of ~th equal to or less than the best comparable LPE DH lasers. In addition, note in fig. 2 that Al~Gai._~AsGaAs DH lasers with GaAs active regions as small as ‘—‘200 A have been grown by MOCVD. These lasers exhibit quantum size effects as a consequence of the confinement of electrons and holes to the extremely thin active region [10 16]. These quantum size effects are potentially of great practical importance and will be discussed in more detail below in section 4. Conventional DII laser structures to be used for continuous-wave (CW) operation at room temperature employ ~As, x Shown = 0.30 confinementtypically layers and GaAs Al~Ga1 active regions. in fig. 3 is the experimental dependence of ~th upon d for such DII structures grown by MOCVD, LPE, and molecular-beam epitaxy (MBE) [17]. These data are for the best reported devices produced by these three III V materials technologies and clearly indicate the excellent quality of MOCVD device structures for conventional DII lasers, In addition to DII lasers with GaAs active regions. MOCVD has been used to produce low-threshold broad-area DII lasers with Al~Ga1 ~As (0.08 y ( 0.12) active regions [18]. These devices emit in ‘~
0.1
.
CURVE b 4800 A/cm2/.Lm) I
I
I
I
I
I
0.2 0.3 0.4 0.5 0.6 0.7 ACTIVE LAYER THICKNESS, d, (sm)
Fig. 3. Variation of ~th upon d for Al~Gai ~As GaAs broad-area DH lasers having x ‘— 0.3. Data points are the best results reported for MOCVD, MBL. and LPE DH lasers. Curves are also shown for the dependence calculated from two different theoretical models [171.
the spectral region near X = 8200 A, a region where a local minimum occurs in the optical loss spectrum of many state-of-the-art optical fibers. For example, injection lasers having Al 0 13Ga077As active regions with d = 930 A and Al0 60Ga0 40As confining layers have been grown by MOCVD. These lasers emit at X = 7990 A and have 2pulsed threshold densities (device length, Lcurrent = 520 pm) [4]. as low as 760 A/cm These results established that MOCVD Al~Ga 1 ~As thin films are of high optical quality and are suitable for use in the active portion of optical devices. 3.2. CW stripe-geometry lasers The broad-area pulsed DII lasers described above are useful for evaluating DH laser material but for most applications, devices capable of CW operation at room temperature or above are required. Lasers that fulfill this requirement generally have a narrow (‘--‘2 25 pm) stripe geometry formed perpendicular to the
216
RD. Dupuis /AlGaAs GaAs lasers grown by MOCVD
Fabry Perot laser facets. Several types of these conventional stripegeometry DH laser structures have been fabricated from MOCVD DH laser wafers, The first CW MOCVD lasers were mesa-stripe-geometry lasers that employed an etched mesa to reduce current spreading parallel to the junction plane (19]. Other types of stripe-geometry AlGaAs GaAs DH MOCVD lasers have been repcrted, e.g. planar Si02-isolated stripe-geometry lasers [20]. DH laser devices 300 pm long with 20 pm wide stripe contacts exhibited 120 mA CW threshold currents. One of these lasers was operated CW for 1400 h with only a 19% increase in ~th [20]. Planar stripe-geometry CW lasers having Al~Gai ~As 0.14 ~ y ~ 0.16 active regions have also been grown by MOCVD [21]. These devices emitted in the wavelength range 7800 ~ X ~ 7600 A and were operated for more than 300 h at room temperature. These results for CW MOCVD lasers clearly demonstrate the capabilities of the MOCVD process to produce CW AlGaAs lasers. The first published data on the lifetime of CW MOCVD lasers (actually taken on a device with a multiple-quantum-well active region to be discussed below) [22] showed that lifetime > 3000 h at 26°Ccould be achieved for such devices. However, for many applications CW 26°C lifetimes greater than iU~h and as long as 106 are required. Future work will undoubtedly concentrate on the study of the reliability of MOCVD CW lasers.
4. Novel MOCVD lasers structures Many of the properties of the conventional planarative-region DII laser structure, e.g., linearity of the light-versus current curve, can be greatly improved by employing alternate active region structures, e.g., non-planar or multiple thin layer active regions. MOCVD has been used to grow such novel device structures and lasers from these Some materials have exhibited manyfabricated unique properties. of these novel MOCVD laser structures are described below. 4.1. Single-mode stripe geometry lasers
Many of the important applications of injection lasers require an optical linewidth that is much less
than 1 .0 A. This requirement can only be met by a laser that is operating in a single longitudinal and transverse mode. This, in turn, requires a device structure that provides for stable control of the optical gain region over a wide range of operating conditions. A number of such mode-stabilized Fabry Perot laser structures have recently been demonstrated. The feature common to all these structures is the provision for stabilization of the optical field in the cavity by the use of special waveguide geometry. The stabilization of the transverse parallel and transverse perpendicular mode results in single-longitudinal-mode owing to the fact that in a CW injection laser, the recombination emission is homogeneously broadened [23] and consequently, the spectral line width narrows as the drive current is increased above threshold [24]. At higher injection levels, the emission becomes concentrated in a single longitudinal mode. Typically, the stabilization of the transverse parallel and transverse perpendicular cavity modes is accomplished by employing a waveguide structure that has a built-in change in the complex index of refraction, n, in the dimension parallel to the junefinn. This built-in change in the index must be larger than the changes in n that are the result of gainguiding and nonuniform injected carrier distributions. Single-longitudinal-mode operation of CW Al~Gai ~As GaAs DH lasers that employ a nonplanar waveguide structure grown by MOCVD has been reported [25 28]. The first such non-planar MOCVD laser structure was the channel-guide (CG) structure shown schematically in fig. 4 [25]. This channel-guide laser structure is formed by first etching ‘--‘3600 A deep and ‘—‘8 pm wide channels parallel to the (011) direction in a (l00)GaAs : Si substrate. Five epitaxial layers are then grown in a single MOCVD growth run. The epitaxial structure is “—0.3 pm GaAs : Se (n = 1 X 1018 cm 3)/”-’ 3)/~~~ 1.4 pm Al0 27Ga0 73As : Se (n = 1 X 1018 csn 640 A GaAs : undoped (n < 1 X 1015 cm3)/——l.2 pm Al 0 27Ga0 73As : Zn (p = 5 X 1O~ cm pm GaAs: Zn (p = 2 X l0’~cm 3). The stabilization of the transverse parallel mode in this cavity is provided by the changes iii n that occur at the bends of the optical waveguide. These bends are the result of overgrowth of the channel etched in the substrate. Significant optical losses
RD. Dupuis / AIGaAs GaAs lasers grown by MOCVD
____________________
217
.—Cr-Au0.5~m
——~
3) 1 pm p + -GaAs (p = 2 x 1019 Cm’ P-GaAIAs (P — 5 x 1017 Cm-3) 1 pm
3500A
__~_.j~-~
—~i~___—GaAs (UNDOPED) 700 A —N-GaAIAs (N —n-GaAs (n
=
=
1 x 1018 Cm-3) 1.4 pm
1 x 1018 Cm-3)
Fig. 4. Schematic cross Section of a channel-guide DII laser grown by MOCVD.
occur for cavity modes that have large amplitudes in these regions. Consequently, high-order transverseparallel optical modes wifi be suppressed and the lowest optical loss will occur for a cavity mode that has a single maximum in the central portion of the waveguide. Since this mode will also experience the largest gain, it would be expected to be the dominant operating mode. This is indeed observed for these CG lasers over a wide range of CW and pulsed current levels [25]. Another extremely important feature of CG laser operation is that they exhibit a linear dependence of light output upon drive current. Linear, kink-free light versus current (L I) curves have been measured for CG lasers up to 10 mW/facet under CW conditions and up to 100 mW/facet under pulsed conditions [25]. If substrates with narrower etched channels are employed for the growth of CC laser structures, the non-planar active region becomes “V” shaped. Recently, such V-DH lasers have been reported that exhibit CW threshold currents ‘-‘-‘20 mA [27] for devices grown in channels ‘—‘1 2 pm wide These devices also operate in a single longitudinal mode [27]
Another type of single mode DII laser structure employs a two-step growth process over a channel etched in a portion of the layered laser structure
~ ing of this device structure is shown in fig. 5. This A1GaAs GaAs self-aligned structure is grown in two
separate MOCVD growth sequences. First, a five layer double heterostructure is grown as follows: (1) 1.5 3.0 pm n-GaAs; (2) 2.0 prri n-Al 0 35Ga0 65As; (3) 700 1000 A undoped GaAs; (4) 0.2 0.6 pm p-Al0 35Ga0 65As; (5) 0.4 0.5 pm n-GaAs. The wafer is then removed from the reactor and a 4 5 pm wide channel is etched in the top surface of the wafer down into the p-Al0 35Ga0 65As confinement layer [26]. The second growth consists of two layers: (1) 1.8 pm p-Al0 35Ga065As and (2) 0.15 pm ptGaAs. The n-GaAs layer that was etched through to provide the channel acts as a current blocking layer and also provides a built-in change in n owing to the increased optical losses that a cavity mode experiences in the region of the waveguide near this layer. Such self-aligned MOCVD laser structures also operate in a single longitudinal mode [261.
-
/
GaAs
‘—p AGaAs ‘—n
GaAs
____________________________
i—p
~
‘—UNDOPED GaAs
-
‘-N
A!GaAS AIGaAS
-
__________________
GaAs
~SUBSTRATE
Fig. 5. Schematic diagram of a self-aligned DII laser ture grown by a two-step MOCVD process.
struc
V V V V V V V V V V V V V V
4 4 V V V V V V V V
218
RD. Dupuis /AIGaAs GaAs lasers grown by MOCVD
Another type of non-planar self-aligned MOCVD laser structure employs growth over an etched and an
Si02 mask used to form the channel [28]. This device structure consists of four epitaxial layers grown in a one-step sequence over a —‘8 pm wide channel
masked by an Si02 layer. The layers are: (1) 1 pm n-Al0 3Ga0 7As; (2) 900 A undoped GaAs; (3) 1.1 pm p-Al0 3Ga0 7As; (4) 0.16 pm p~-GaAs[28]. The active region in this structure is both non-planar and non-uniform in thickness. The multiple-layer structure that is deposited upon the Si02 mask is polycrystalline and exhibits high electrical resistance. As a consequence, contact metallization can be applied to the entire wafer surface and the current will be injected only in the single-crystal layers grown in the unmasked channel. These devices operate CW at currents ‘--‘50 mA de but do not Operate in a single longitudinal mode [28].
4.2.
E9 (GaAs)
Ethi
E5 ~
Ehhl
::
~
~Ev15%~
E9
Fig. 6. Diagram (not to scale) 01 the quantum levels of conduction-band electrons and valence-band light and heavy holes confined to a Al~Gai ~As GaAs quantum well.
Other types of laser structures employing MOCVD combined with LPE
The MOCVD process has also been combined with LPE to produce other types of laser structures. MOCVD layers have been used to bury LPE-grown mesas [30] and as blocking layers in LPE-grown channeled-substrate-planar (CSP) laser structures [31]. Since these structures do not employ active regions grown by MOCVD, they will not be discussed further here.
4.3.
E9 (GaAIAs)
Quantum-well hetero structure lasers
layer. The quantum well thickness is L~ and the conduction band and valence band differences are 85~LEg, respectively [32]. The energy levels of the electron confined-particle states in the conduction band are labelled E1, etc. The light-hole valence band energy states are labelled E1~1,etc., while the heavy-hold states are labeled Ehh ~‘ etc. Radiative recombination can occur only between electron and hole states having the same quantum number (i.e., = 0), thus the output spectra of such a structure will consist of a series of discrete lines characteristic of the quantum-well confined-particle states.
The output characteristics of most conventional Al~Gai ~As GaAs DH lasers are determined by the bulk properties of the GaAs and Al~Gai ~As
For lasers with having single active regions of thickness d < 400 A, these quantum size effects play a dominant role in the lasing properties. Such quan-
materials that forni the laser structure. However, if the active region thickness, d, is smaller than ‘--‘500 A, the laser characteristics are influenced by quantummechanical effects, in addition to the properties of “bulk” GaAs and AJ~Ga1 ~As. These quantum-size effects [15] result from the confinement of electrons and holes to the extremely thin active region of such a DII laser. The energy levels of these electrons and holes are quantized, much that same as the classical “particle in a box”. This is shown schematically in fig. 6 for the case of a single GaAs quantum well bounded on each side by a Al~Ga1 ~As confining
turn-well heterostructure (QWII) lasers having active regions as thin as d 200 A have been grown by MOCVD [ill. Laser structures having coupled multiple-quantumwell active regions separated by thin (‘--‘10 100 A) higher-bandgap A1GaAs barrier layers can also be grown by MOCVD. Studies of such QWH opticallypumped and injection lasers have established that these structures exhibit a variety of unique and potentially useful optical and electrical properties [10 14,33 46]. The growth of such multiple thin-layer laser struc-
RD. Dupuis/A1GaAs
GaAs lasers grown by MOCVD
tures requires a materials technology that is capable of producing high-quality GaAs and Al~Ga1 ~As
Energy (eV) 1.45 I
epitaxial layers as well as abrupt Al~Ga1~As GaAs heterojunctions having alow densityof defects. Analysis of single- and multiple-layer MOCVD quantum-well heterostructures by Auger electron spectroscopy and simultaneous argon.ion sputter etching has established that the chemical-interface width of MOCVD Al~Gam ~As GaAs heterojunction is ~ 20A [7,47]. In particular, analysis of an Al045Ga055As GaAs multiple-quantum-well structure grown by MOCVD showed that the chemical interface width was ~ 17 A
219
1.50 1.55 1.60 I Al~Gai ~As GaAs (1+2)
L~ bOA t
311pm
300 K .~
[471. The high quality of MOCVD A1GaAs GaAs quantum-well heterostructures is evidenced by the fact that pulsed and CW room-temperature laser operation of optically-pumped and injection single- [10,11] and multiple- [33,37] quantum-well lasers grown by MOCVD have been reported. The low thresholds and high differential quantum efficiencies measured for
MOCVD ting these todevices note QWH further that materials the substantiate first [14].CW Inthe room-temperature fact, high itlasers quality is interesof grown singleThe by and 300 MOCVD rimultiple-quantum-well K emission [39,45]. spectra (pulsed diode and CW) of were an optically grown has by MOCVD ispumped shown quantum-well in fig. 7 [13]. structure This structure two GaAs quantum wells having a thickness L Z ““ 100 A. The curve (a) shows CW laser operation on the n = 1 1’ confined-particle transitions at an mcident power of ‘-—iü~W/cm2 - At higher pulsed excitation levels (b), laser operation occurs on both the n = 1 1’ and n = 2 2’ transitions [13]. Ouantumwell injection lasers have also operated CW on con-
fined-particle transitions at energies above the GaAs bandedge [391. The properties of QWH lasers are unique in many significant respects. Typically, MOCVD QWH A1GaAs GaAs lasers exhibit low thresholds, high-energy spectral emission (relative to bulk GaAs), and high external differential quantum efficiencies [37,39,45] other desirable features of diode lasers having QWH
active regions are that such devices tend to operate in a single longitudinal mode [451Multiple-quantum-well DH laser structures grown by MOCVD exhibit yet another interesting feature a strong coupling of LU phonons to the large con-
~ .~
—
I fl
.~ u 8.8— Jbm2(CW) ~ a) I I I 1 8.41’
I
I
I
8.0 I 2 2’ 7.6 3A) Wavelength Fig. 7. Emission spectra (300 K)(1001 an optically-pumped MOCVD multiple-quantum-well laser. The calculated confined-particle quantum-well transition energies are shown as dark (n, e —~hh) and open (n’, e —~ lh) bars along the wavelength axis. Laser operation occurs on confinedparticle states for both CW and pulsed excitation.
fined-particle electron population in the conduction band of such QWH lasers [41 46]. This coupling
leads to optical emission at energies corresponding to a quantum-well confined electron hole state shifted down in energy by an amount equal to one or more LU phonons. In fact, laser operation of optically-pumped multiple-quantum-well heterostructures has been achieved on phonon-assisted confinedparticle recombination transitions at energies as much as four LU Phonons from the corresponding
confined-particle
energy
level
[14,44,45,49,50].
Thus phonon interaction (principally with LO phonons) leads to profound changes in the laser spectra of such structures.
220
R.D. Dupuis/AIGaAs GaAs lasers grown by MOCVD
The MOCVD process has also been used to grow quantum-well heterostructures consisting of thin GaAs wells (Lz 50 A) and thin AlAs barriers (LB 50 A) [51,52]. Such structures exhibit a greatly reduced phonon coupling in their lasing characteristics owing to the elimination of alloy clustering in the pure AlAs barriers [50]. These results have shown that alloy clustering in Al~Ga 1 ~As, 0
5. MOCVD CW laser lifetimes
longitudinal mode for the entire length of the test (t > 3200 h). While these results were obtained on unoptimized devices, they indicate that injection lasers with useful lifetimes can be grown by MOCVD. Further detailed studies are needed to identify any defects in MOCVD materials that might limit laser lifetimes.
6. Summary and conclusions Metalorganic chemical vapor deposition has been shown to be a viable materials technology for the growth of a wide variety of A1GaAs GaAs laser structures. The performance characteristics of MOCVD conventional DII lasers having both GaAs and A1GaAs active regins have been shown to be equal to or better than the characteristics of similar DH laser structures grown by liquid-phase epitaxy and molecular-beani epitaxy. In addition, the MOCVD process has been shown to be useful in the growth of novel DII laser structures that exhibit room-temperature CW single-mode lasers having linear light-output versus current characteristics. High-quality single- and multiple-quantum-well Dl-1 laser structures have also been grown. These structures consist of epitaxial layers as thin as —-‘10 A and clearly demonstrate the capability of the MOCVD process to grow ultra-thin layers. Furthermnore, the lasing characteristics of these structures
imply that the MOCVD A1GaAs GaAs heterojuncIt is of great practical interest to determine the CW lifetimes of MOCVD injection lasers. Lifetimes ~ i0~ h at 300 K are generally required for practical laser applications. While only preliminary data on the lifetime of MOCVD lasers has been published [20 22], the performance of these devices has been exceedingly promising as mentioned above. For example, constant-current continuous operation at roomtemperature (“--‘26°C) has been achieved for CW stripe-geometry mnultiple-quantum-well lasers for times greater than 3200 h [22]. The diodes of this study had an “active region” consisting of six GaAs quantum wells with a thickness “—‘120 A separated by five Al0 30Ga0 70As barriers also “-‘120 A thick. No significant degradation in the output of one of these devices was observed during the first —‘700 h of CW operation. In addition, this device lased in a single
tions are extremely uniform and abrupt and contain to large interface non-radiative recombination centers.
The promising (although preliminary) data on CW 300 K MOCVD lasers published to date indicates that useful DH lasers can be made by MOCVD and that there are no fundamental barriers to the future development of long-lived MOCVD lasers. These results for MOCVD AlGaAs GaAs layers, and the inherent advantages that the MOCVD process offers in the growth of large area, uniform epitaxial layers indicate that metalorganic chemical vapor deposition is a very promising technology for the large-scale growth of AlGaAs GaAs lasers and other heterojunction devices.
RD. Dupuis
/ AIGaAs
References [1] ll.M. Manasevit, Appl. Phys. Letters 12 (1968) 156. [2] I-or a review, see: H.M. Manasevit, J. Crystal Growth 13/14 (1972) 306; H.M. Manasevit, J. Crystal Growth 22 (1974) 125. [31 RD. Dupuis and P.D. Dapkus, Appl. Phys. Letters 31 (1977) 466. [4] R.D. Dupuis and P.D. Dapkus, IEEE J. Quantum Electron. QL-15 (1979) 128. [5] S.D. Hersee, J.P. Hirtz, Bui-Dinh-Vuong, J.P. Duchemi M. Bonnet, G. Mesquida and A. Parent, 7th IEEE Intern. Semiconductor Laser Conf., Brighton, UK, 1980. [6] RD. Dupuis and PD. Dapkus, Appl. Phys. Letters 32 (1978) 473. [7] R.D. Dupuis, L.A. Moudy and PD. Dapkus, in: Proc. 7th Intern. Symp. on GaAs and Related Compounds, St. Louis, 1978, Inst. Ph~s. Cont. Ser. 45 (Inst. Phys., London, 1979) ~ 1. [8] E.J. Thrush, P.R. Selway and GD. Henshall, Electron. Letters 15 (1979) 156. [9] H. Kressel and M. Ettenberg, J. AppI. Phys. 47 (1976)
3533. [10] R.M. Kolbas, N. Holonyak, Jr., R.D. Dupuis and P.D. Dapkus, Pisma Zh. Tekh. Hz. 4 (1978) 69 [Soviet Tech. Phys. Letters 4 (1978) 28]. [11] RD. Dupuis, P.D. Dapkus, N. Unlonyak, ir., L.A. Rezek and R. Chin, Appl. Phys. Letters 32(1978) 295. [12] N. Holonyak, Jr., R.M. Kolbas, W.D. Laidig, B.A. Vojak, RD. Dupuis and P.D. Dapkus, in: Proc. 7th Intern. Symp. on GaAs and Related Compounds, St. Louis, 1978, Inst. Phys. Cont. Ser. 45 (Inst. Phys., London, 1979) p. 387. [13] RD. Dupuis, PD. Dapkus, R.M. Kolbas and N. Holonyak, Jr., IFFE i. Quantum Electron. QE-15 (1979) 756. [14] N. Holonyak, Jr., R.M. Kolbas, R.D. Dupuis and PD. Dapkus, IFLE J. Quantum Electron. QE-16 (1980) 170. [15] For a discussion of quantum-size effects in III V semiconductor heterostructures, see R. Dingle, in: I estkorperprobleme XV, Advances in Solid State Physics, Ed. Iii. Qucisser (Pergamon Vieweg, Braun21. sLhweig, Germany, 1975) p. (16) A discussion of non-equilibrium carrier populations in quantum-well structures is given by K. Hess and N. Holonyak, ir., Phys. Today 33 (1980) 40. [17] W.T. Tsang, Appl. Phys. Letters 36(1980)11. [18] R.D. Dupuis and PD. Dapkus, Appl. Phys. Letters 31 (1977) 839. [19] RD. Dupuis and PD. Dapkus, Appl. Phys. Letters 32 (1978) 406. 120] E.J. Thrush and J.E.A. Whiteaway, Electron. Letters 15 (1979) 666. [21] Y. Mon and N. Watanabe, Electron. Letters 16(1980) 285. [22) R.D. Dupuis, AppI. Phys. Letters 35 (1979) 311.
GaAs lasers grown by MOCVD
221
[23] T.L. Paoh and i.E. Ripper, Appl. Phys. Letters 25 (1974) 744. [24] For a discussion of spectral narrowing of homogeneously broadened transitions, see: W.V. Smith and P. D. Sorokin, in: The Laser (McGraw-Hill, New York, 1966). [25] RD. Dupuis and P.D. Dapkus, AppI. Phys. Letters 33 (1978) 724. [26] J.J. Coleman and P.D. Dapkus, Appl. Phys. Letters 37 (1980) 262. [27] Y. Mori, 0. Matsuda, K. Morizane and N. Watanabe, Electron. Letters 16(1980). [28] D. Fekete, RD. Burnham, DR. Scrifres, W. Streifer and RD. Yinghing, Appl. Phys. Letters 38 (1981) 607. [29] N. Nishi, M. Yan”, Y. Nishitani, Y. Akita and M. Takusagawa, Appl. Phys. Letters 35 (1979) 232. [30] II. Veenvliet, C. van Opdorp, R.P. Tijburg and J.P. Andre, IEEE J. Quantum Flectron. QE-l5 (1979) 762. (31] J.P. Noad, CM. Look and A.J. Springthorpe, Llectron. Letters 16 (1980) 685. [32] R. Dingle, W. Wiegmann and CU. Henry, Phys. Rev. Letters 33(1974) 827. [33] N. Holonyak, Jr., R.M. Kolbas, RD. Dupuis and P.D. Dapkus, Appl. Phys. Letters 33 (1978) 73. [34] R.D. Dupuis, PD. Dapkus, R.M. Kolbas and N. Holonyak, Jr., Solid State Commun. 27 (1978) 531. [35] R.D. Dupuis, P.D. Dapkus, R.M. Kolbas, N. Holonyak, Jr. and H. Shichijo, AppI. Phys. Letters 33 (1978) 596. [36] 1-1. Shichijo, R.M. Kolbas, N. Flolonyak, Jr., RD. Dupuis and PD. Dapkus, Solid State Cominun. 27 (1978) 1029. [37] N. Holonyak, Jr., R.M. Kolbas, W.D. Laidig, BA. Vojak, RD. Dupuis and P.D. Dapkus, i. Appl. Phys. Letters 33(1978) 737. [38] N. Holonyak, Jr., R.M. Kolbas, E.A. Rezek, R. Chin, RD. Dupuis and PD. Dapkus, J. Appl. Phys. 49 (1978) 5392. [39) RD. Dupuis, PD. Dapkus, R. Chin, N. Flolonyak, Jr. and SW. Kirchoefer, Appl. Phys. Letters 34 (1979) 265. [40] RD. Dupuis, P.D. Dapkus, N. Ilolonyak, Jr., R.M. Kolbas, W.D. Laidig and BA. Vojak, Pisma Zh. Tekh. Fiz. 5 (1979) 132 [Soviet Tech. Phys. Letters 5 (1979) 52]. [41] N. Holonyak, Jr., R.M. Kolbas, W.D. Laidig,M. Altareili, RD. Dupuis and PD. Dapkus, Appl. Phys. Letters 34 (1979) 501. [42] B.A. Vojak, SW. Kirchoefer, N. Holonyak, Jr., R. Chin, R.D. Dupuis and P.D. Dapkus, J. AppI. Phys. 51) (1979) 5810 [431 B.A. Vojak, N. Holonyak, Jr., R. Chin, E.A. Rezek, R.D. Dupuis and P.D. Dapkus,J. AppI. Phys. 50 (1979) 5835. [44] R.M. Kolbas, N. Holonyak, Jr., B.A. Vojak, K. Hess,
S SI
SI
55 55 S5 S5 55 V V V
222
[45] [46]
[47] [48] [49]
R.D. Dupuis /AlGaAs GaAs lasers grown by MOCVD M. Altarelli, R. D. Dupuis and P.D. Dapkus, Solid State Commun. 31(1979)1033. RD. Dupuis, P.D. Dapkus, N. Holonyak, Jr. and R.M. Kolbas, App!. I’hys. Letters 35 ~l979j 487. N. Holonyak, Jr., R.M. Kolbas, W.D. Laidig, BA. Vojak, K. Hess, R.D. Dupuis and P.D. Dapkus, J. AppI. Phys. 51(1980)1328. R.D. Dupuis, PD. Dapkus, C.M. Garner, C.Y. Su and W.E. Spicer, Appl. Phys. Letters 34 (1979) 338. M. Ettenberg, Ci. Nuese and H. Kressel, J. Appl. Plsys. 50(1979)2949. K. Hess, Appl. Phys. Letters 35 (1979) 484.
[50] B.A. Vojak, N. Holonyak, Jr., W.D. Laidig, K. Hess, J. J. Coleman and P.D. Dapkus, J. App!. Phys. 52 (1981) 959. [51] N. Holonyak, Jr., W.D. Laidig, B.A. Vojak, K. Hess, ii. Coleman, P.D. Dapkus and J. Bardeen, Phys. Rev. Letters 45 (1980) 1703. [52] J.J. Colensan, PD. Dapkus, W.D. Laidig, B.A. Vojak and N. Holonyak, Jr., Appl. Phys. Letters 38 (1981) 63. [53] B.A. Vojak, W.D. Laidig, N. Holonyak, Jr. M.D. Camras, J.J. Coleman and P.D. Dapkus, J. AppI. Phys. 52 (1981)621.
S
213
AIGaAs GaAs LASERS GROWN BY METALORGANIC CHEMICAL VAPOR DEPOSITION R.D. DIJPUIS
—
A REVIEW
*
Bell Laboratories, Murray Hill, New Jersey 07974, USA
High quality double-heterostructure (DH) lasers having excellent device characteristics have been grown by metalorganic chemical vapor deposition (MOCVD), as reported by a number oflaboratories. In addition to devices employing conventional planar DH laser geometries, high-performance lasers having unique device structures have also been demonstrated. This paper will review recent world-wide progress in the application of MOCVD to the growth of AIGaAs GaAs laser structures.
1. Introduction The metalorganic chemical vapor deposition process is an open-tube vapor-phase thin-film crystal growth technology that employs metal alkyls (or organometallics e.g., trimethylgallium, (CH3)3Ga), and hydrides (e.g., arsine, AsH3), as sources of the elements of which the thin film is to be made. The metalorganic chemical vapor deposition (MOCVD) process was first demonstrated by Manasevit in 1968 [1]. Much of the early research on the growth of III V compound semiconductor films by this vapor-phase materials technology was concentrated on the growth of heteroepitaxial thin films of various III V materials upon insulating oxide substrates, e.g., A1203 and BeO [2]. More recently, however, the primary interest in MOCVD has been for the growth of semiconductor thin films on closely-lattice-matched semiconductor substrates. for the fabrication of III V semiconductor heterojunction devices such as lasers, detectors, and field-effect transistors (FET’s). This paper wifi review the development of the MOCVD materials technology for one of these specific device applications the MOCVD growth of Al~Gai~As GaAs heterojunction structures for lasers. In addition to conventional broad-area and stripe-geometry double-heterostructure (DH) lasers, many novel —
*
Work was done while Anaheim, California, USA.
at Rockwell International,
0022-0248/81/0000 0000/$02.50 © 1981 North-Holland
laser structures have been grown by MOCVD, providing improved and unique device performance characteristics.
2. MOCVD materials growth for lasers The majority of the MOCVD research for A1GaAs DH lasers has employed trimethylgallium (TMGa) and trimethylaluminum (TMA1) as sources of the column III metals and arsine (AsH3) as the source of the column V element As. The doping of the pand n-type AlGaAs and GaAs epitaxial layers required for an injection laser structure is typically accomplished by employing diethylzinc (DEZn) and H2Se, respectively [3]. The epitaxial growth on an appropriate substrate is accomplished in an open-tube reactor by the pyrolysis of vapor-phase mixtures of these compounds at 650 750°C in H2 at (or slightly below) atmospheric pressure. For Al~Ga1~Asalloy films, the alloy composition is controlled by the relative initial partial pressures of TMGa and TMAI established in the reactor during growth. Similarly, the n-type Se and ptype Zn doping levels in the epitaxial films are controlled by the initial partial pressures of the H2 Se and DEZn sources. A schematic diagram of the atmosphericpressure MOCVD reactor that Dupuis and Dapkus have employed for the growth of DH lasers is shown in fig. 1 [3,4]. Since the metalorganics TMGa, TMA1, and DEZn are liquids near room tem-
214
R.D. Dupuis/AIGaAs GaAs lasers grown by MOCVD MASS FLOW CONTROLLERS
2
QUARTZ REACTOR ~WER
VACUUM
Fig. 1.
AsH~
H
EXHAUST
p
.L
I OEZn
~
~-1~J
technology make it wellsuited for the growth of uniform large-area submicron epitaxial thin films required for the cost-effective production of DH 2 in areaGaAs have beenlaser grown by lasers. In fact, Al~Ga1~As DH wafers as larger than 11 cm ::on:ntio~al DH lasers grown by MOCVD
~~~FIED
SUSCEPTOR
These characteristics of the MOCVD materials
DEWAR FLASK
3.1. Pulsed broad-area lasers
Schematic diagram of MOCVD reactor. The
perature with relatively high vapor pressures, they can be readily transported into the reactor by bubbling a H2 carrier gas through the liquid. The hydrides AsH3 and H2 Se are gases at room temperature and are usually supplied as mixtures in H2. The graphite susceptor is heated by an RF induction coil that is external to the quartz reactor chamber. The susceptor is usually rotated during growth. While most MOCVD lasers have been grown by this atmospheric-pressure MOCVD process, Hersee et al. have reported the growth of AlGaAs DH lasers by low-pressure MOCVD (LPMOCVD) using a horizontal reactor tube [5]. During growth, a roughing pump is used to reduce the pressure in the reactor to —~l00Torr. Layer thickness control and uniformity are excellent for MOCVD epitaxial layers. This is important for the reproducible growth of high-yield DH laser wafers. Even using deposition conditions corresponding to growth rates ~‘3000 A/mm, layers as thin as ‘-~50A can be reproducibly grown. While the uniformity of such a thin layer is not easily measured, SEM measurements of the thickness of ~1 pm thick MOCVD epitaxial Al~Gai_~As layers have shown that the thickness variation is less than ±1.5%over greater than 80% of the length of a 4.3 cm long bar cleaved from an MOCVD DH laser wafer [6] In addition to layer thickness control and uniformity, the MOCVD process offers excellent control and uniformity of the growth rate, doping density, and alloy composition of A1GaAs layers [7], as well as low interface recombination velocities at A1GaAs GaAs heterojunctions [8]. -
conventional Al~Ga
1 ~As GaAs doubleheterostructure laser typically employs a three-layer symmetrical dielectric waveguide consisting of a central GaAs “active region” and two Al~Ga1 ~As “passive” cladding or confining layers. Such structures are readily grown by MOCVD. In practive, five epitaxial layers are grown. The first layer is typically a 0.3 1 .0 pm GaAs: Se n-type buffer layer. Then a 1.0 2.0 pm Al~Gai_~As : Se n-type confining layer, a 0.01 0.3 pm undoped GaAs active layer. a 1.0 2.0 pm Al~Ga1~As : Zn p-type confining layer, and an 0.3 1.0 pm GaAs: Zn p-type “cap” layer are sequentially deposited in a single MOCVD growth cycle [3]. Broad-area injection lasers with cleaved facets and sawed sides are then fabricated from the wafer. Typical dimensions are 200 600 pm between Fabry Perot laser facets and 200 pm between sawed sides. These devices are then tested under pulsed current excitation employing ‘~200ns pulses at repetition rate ~-‘l kHz. The spectral output is measured as a function of peak drive current and the current required for the onset of laser operation is determined. The dependence of the threshold current density, ~th, of broad-area MOCVD DH lasers upon the active layer thickness, d, and the alloy composition, x, of the Al~Gai ~As confining layers is shown in fig. 2. The closed data points give the lowest values of ~th measured for broad-area MOCVD lasers [6]. For comparison, the lowest reported values measured for comparable Al~Ga1~As DH lasers grown by liquidphase epitaxy (LPE) are also shown (open data points) [9]. The lines of fig. 2 show the dependence of ~th upon d and x determined by an approximate calculation [6]. As shown in fig. 2, MOCVD DH
RD. Dupuis / AIGaAs GaAs lasers grown by MOCVD ~~.1
3000
MO-CVD A x = 0.27 . x =0.32 • x —0.50
E
~ 2500 ~
~
(A
2000
-x
= =
ILL
~ ~
0.20 0.27
0
0
L PE — 0 25 ~x =045~ ~x = 0.50 = 0.55 ~x = 0.68
215 2
LPE
36 380 X 200 0:25-0.35 :500x 100 MO - CVD V,~ x = 0.27 -0.30 .—~5O0X 200 MBE • x =0.30 ‘375 x 200 0
4.0 E
~ILI 1500 W//X=0.50 W/ ~ ~ /
~
x
DIODEAREA(~m)
0
X=
-,
>~
I-
c_,l 000I’~
A
0
2~
3.0
-
(AVERAGED ~th) CURVE a 2~m)
~ =
500 I
2.0 I
~-*=
/
5160 A/cm
~
GaAs 00500 pulsed ACTIVE broad-area 1000 LAYER 1500 THICKNESS, DH lasers 2000 d (A) grown 3000 ?500 by MOCVD Fig. Dependence and x for Al~Gai xAs (open2. points) and of for~ththeupon bestd comparable lasers grown by LPE (closed points).
/
1.0- ~% ~ (—~=
I
0
broad-area lasers exhibit values of ~th equal to or less than the best comparable LPE DH lasers. In addition, note in fig. 2 that Al~Gai._~AsGaAs DH lasers with GaAs active regions as small as ‘—‘200 A have been grown by MOCVD. These lasers exhibit quantum size effects as a consequence of the confinement of electrons and holes to the extremely thin active region [10 16]. These quantum size effects are potentially of great practical importance and will be discussed in more detail below in section 4. Conventional DII laser structures to be used for continuous-wave (CW) operation at room temperature employ ~As, x Shown = 0.30 confinementtypically layers and GaAs Al~Ga1 active regions. in fig. 3 is the experimental dependence of ~th upon d for such DII structures grown by MOCVD, LPE, and molecular-beam epitaxy (MBE) [17]. These data are for the best reported devices produced by these three III V materials technologies and clearly indicate the excellent quality of MOCVD device structures for conventional DII lasers, In addition to DII lasers with GaAs active regions. MOCVD has been used to produce low-threshold broad-area DII lasers with Al~Ga1 ~As (0.08 y ( 0.12) active regions [18]. These devices emit in ‘~
0.1
.
CURVE b 4800 A/cm2/.Lm) I
I
I
I
I
I
0.2 0.3 0.4 0.5 0.6 0.7 ACTIVE LAYER THICKNESS, d, (sm)
Fig. 3. Variation of ~th upon d for Al~Gai ~As GaAs broad-area DH lasers having x ‘— 0.3. Data points are the best results reported for MOCVD, MBL. and LPE DH lasers. Curves are also shown for the dependence calculated from two different theoretical models [171.
the spectral region near X = 8200 A, a region where a local minimum occurs in the optical loss spectrum of many state-of-the-art optical fibers. For example, injection lasers having Al 0 13Ga077As active regions with d = 930 A and Al0 60Ga0 40As confining layers have been grown by MOCVD. These lasers emit at X = 7990 A and have 2pulsed threshold densities (device length, Lcurrent = 520 pm) [4]. as low as 760 A/cm These results established that MOCVD Al~Ga 1 ~As thin films are of high optical quality and are suitable for use in the active portion of optical devices. 3.2. CW stripe-geometry lasers The broad-area pulsed DII lasers described above are useful for evaluating DH laser material but for most applications, devices capable of CW operation at room temperature or above are required. Lasers that fulfill this requirement generally have a narrow (‘--‘2 25 pm) stripe geometry formed perpendicular to the
216
RD. Dupuis /AlGaAs GaAs lasers grown by MOCVD
Fabry Perot laser facets. Several types of these conventional stripegeometry DH laser structures have been fabricated from MOCVD DH laser wafers, The first CW MOCVD lasers were mesa-stripe-geometry lasers that employed an etched mesa to reduce current spreading parallel to the junction plane (19]. Other types of stripe-geometry AlGaAs GaAs DH MOCVD lasers have been repcrted, e.g. planar Si02-isolated stripe-geometry lasers [20]. DH laser devices 300 pm long with 20 pm wide stripe contacts exhibited 120 mA CW threshold currents. One of these lasers was operated CW for 1400 h with only a 19% increase in ~th [20]. Planar stripe-geometry CW lasers having Al~Gai ~As 0.14 ~ y ~ 0.16 active regions have also been grown by MOCVD [21]. These devices emitted in the wavelength range 7800 ~ X ~ 7600 A and were operated for more than 300 h at room temperature. These results for CW MOCVD lasers clearly demonstrate the capabilities of the MOCVD process to produce CW AlGaAs lasers. The first published data on the lifetime of CW MOCVD lasers (actually taken on a device with a multiple-quantum-well active region to be discussed below) [22] showed that lifetime > 3000 h at 26°Ccould be achieved for such devices. However, for many applications CW 26°C lifetimes greater than iU~h and as long as 106 are required. Future work will undoubtedly concentrate on the study of the reliability of MOCVD CW lasers.
4. Novel MOCVD lasers structures Many of the properties of the conventional planarative-region DII laser structure, e.g., linearity of the light-versus current curve, can be greatly improved by employing alternate active region structures, e.g., non-planar or multiple thin layer active regions. MOCVD has been used to grow such novel device structures and lasers from these Some materials have exhibited manyfabricated unique properties. of these novel MOCVD laser structures are described below. 4.1. Single-mode stripe geometry lasers
Many of the important applications of injection lasers require an optical linewidth that is much less
than 1 .0 A. This requirement can only be met by a laser that is operating in a single longitudinal and transverse mode. This, in turn, requires a device structure that provides for stable control of the optical gain region over a wide range of operating conditions. A number of such mode-stabilized Fabry Perot laser structures have recently been demonstrated. The feature common to all these structures is the provision for stabilization of the optical field in the cavity by the use of special waveguide geometry. The stabilization of the transverse parallel and transverse perpendicular mode results in single-longitudinal-mode owing to the fact that in a CW injection laser, the recombination emission is homogeneously broadened [23] and consequently, the spectral line width narrows as the drive current is increased above threshold [24]. At higher injection levels, the emission becomes concentrated in a single longitudinal mode. Typically, the stabilization of the transverse parallel and transverse perpendicular cavity modes is accomplished by employing a waveguide structure that has a built-in change in the complex index of refraction, n, in the dimension parallel to the junefinn. This built-in change in the index must be larger than the changes in n that are the result of gainguiding and nonuniform injected carrier distributions. Single-longitudinal-mode operation of CW Al~Gai ~As GaAs DH lasers that employ a nonplanar waveguide structure grown by MOCVD has been reported [25 28]. The first such non-planar MOCVD laser structure was the channel-guide (CG) structure shown schematically in fig. 4 [25]. This channel-guide laser structure is formed by first etching ‘--‘3600 A deep and ‘—‘8 pm wide channels parallel to the (011) direction in a (l00)GaAs : Si substrate. Five epitaxial layers are then grown in a single MOCVD growth run. The epitaxial structure is “—0.3 pm GaAs : Se (n = 1 X 1018 cm 3)/”-’ 3)/~~~ 1.4 pm Al0 27Ga0 73As : Se (n = 1 X 1018 csn 640 A GaAs : undoped (n < 1 X 1015 cm3)/——l.2 pm Al 0 27Ga0 73As : Zn (p = 5 X 1O~ cm pm GaAs: Zn (p = 2 X l0’~cm 3). The stabilization of the transverse parallel mode in this cavity is provided by the changes iii n that occur at the bends of the optical waveguide. These bends are the result of overgrowth of the channel etched in the substrate. Significant optical losses
RD. Dupuis / AIGaAs GaAs lasers grown by MOCVD
____________________
217
.—Cr-Au0.5~m
——~
3) 1 pm p + -GaAs (p = 2 x 1019 Cm’ P-GaAIAs (P — 5 x 1017 Cm-3) 1 pm
3500A
__~_.j~-~
—~i~___—GaAs (UNDOPED) 700 A —N-GaAIAs (N —n-GaAs (n
=
=
1 x 1018 Cm-3) 1.4 pm
1 x 1018 Cm-3)
Fig. 4. Schematic cross Section of a channel-guide DII laser grown by MOCVD.
occur for cavity modes that have large amplitudes in these regions. Consequently, high-order transverseparallel optical modes wifi be suppressed and the lowest optical loss will occur for a cavity mode that has a single maximum in the central portion of the waveguide. Since this mode will also experience the largest gain, it would be expected to be the dominant operating mode. This is indeed observed for these CG lasers over a wide range of CW and pulsed current levels [25]. Another extremely important feature of CG laser operation is that they exhibit a linear dependence of light output upon drive current. Linear, kink-free light versus current (L I) curves have been measured for CG lasers up to 10 mW/facet under CW conditions and up to 100 mW/facet under pulsed conditions [25]. If substrates with narrower etched channels are employed for the growth of CC laser structures, the non-planar active region becomes “V” shaped. Recently, such V-DH lasers have been reported that exhibit CW threshold currents ‘-‘-‘20 mA [27] for devices grown in channels ‘—‘1 2 pm wide These devices also operate in a single longitudinal mode [27]
Another type of single mode DII laser structure employs a two-step growth process over a channel etched in a portion of the layered laser structure
~ ing of this device structure is shown in fig. 5. This A1GaAs GaAs self-aligned structure is grown in two
separate MOCVD growth sequences. First, a five layer double heterostructure is grown as follows: (1) 1.5 3.0 pm n-GaAs; (2) 2.0 prri n-Al 0 35Ga0 65As; (3) 700 1000 A undoped GaAs; (4) 0.2 0.6 pm p-Al0 35Ga0 65As; (5) 0.4 0.5 pm n-GaAs. The wafer is then removed from the reactor and a 4 5 pm wide channel is etched in the top surface of the wafer down into the p-Al0 35Ga0 65As confinement layer [26]. The second growth consists of two layers: (1) 1.8 pm p-Al0 35Ga065As and (2) 0.15 pm ptGaAs. The n-GaAs layer that was etched through to provide the channel acts as a current blocking layer and also provides a built-in change in n owing to the increased optical losses that a cavity mode experiences in the region of the waveguide near this layer. Such self-aligned MOCVD laser structures also operate in a single longitudinal mode [261.
-
/
GaAs
‘—p AGaAs ‘—n
GaAs
____________________________
i—p
~
‘—UNDOPED GaAs
-
‘-N
A!GaAS AIGaAS
-
__________________
GaAs
~SUBSTRATE
Fig. 5. Schematic diagram of a self-aligned DII laser ture grown by a two-step MOCVD process.
struc
V V V V V V V V V V V V V V
4 4 V V V V V V V V
218
RD. Dupuis /AIGaAs GaAs lasers grown by MOCVD
Another type of non-planar self-aligned MOCVD laser structure employs growth over an etched and an
Si02 mask used to form the channel [28]. This device structure consists of four epitaxial layers grown in a one-step sequence over a —‘8 pm wide channel
masked by an Si02 layer. The layers are: (1) 1 pm n-Al0 3Ga0 7As; (2) 900 A undoped GaAs; (3) 1.1 pm p-Al0 3Ga0 7As; (4) 0.16 pm p~-GaAs[28]. The active region in this structure is both non-planar and non-uniform in thickness. The multiple-layer structure that is deposited upon the Si02 mask is polycrystalline and exhibits high electrical resistance. As a consequence, contact metallization can be applied to the entire wafer surface and the current will be injected only in the single-crystal layers grown in the unmasked channel. These devices operate CW at currents ‘--‘50 mA de but do not Operate in a single longitudinal mode [28].
4.2.
E9 (GaAs)
Ethi
E5 ~
Ehhl
::
~
~Ev15%~
E9
Fig. 6. Diagram (not to scale) 01 the quantum levels of conduction-band electrons and valence-band light and heavy holes confined to a Al~Gai ~As GaAs quantum well.
Other types of laser structures employing MOCVD combined with LPE
The MOCVD process has also been combined with LPE to produce other types of laser structures. MOCVD layers have been used to bury LPE-grown mesas [30] and as blocking layers in LPE-grown channeled-substrate-planar (CSP) laser structures [31]. Since these structures do not employ active regions grown by MOCVD, they will not be discussed further here.
4.3.
E9 (GaAIAs)
Quantum-well hetero structure lasers
layer. The quantum well thickness is L~ and the conduction band and valence band differences are 85~LEg, respectively [32]. The energy levels of the electron confined-particle states in the conduction band are labelled E1, etc. The light-hole valence band energy states are labelled E1~1,etc., while the heavy-hold states are labeled Ehh ~‘ etc. Radiative recombination can occur only between electron and hole states having the same quantum number (i.e., = 0), thus the output spectra of such a structure will consist of a series of discrete lines characteristic of the quantum-well confined-particle states.
The output characteristics of most conventional Al~Gai ~As GaAs DH lasers are determined by the bulk properties of the GaAs and Al~Gai ~As
For lasers with having single active regions of thickness d < 400 A, these quantum size effects play a dominant role in the lasing properties. Such quan-
materials that forni the laser structure. However, if the active region thickness, d, is smaller than ‘--‘500 A, the laser characteristics are influenced by quantummechanical effects, in addition to the properties of “bulk” GaAs and AJ~Ga1 ~As. These quantum-size effects [15] result from the confinement of electrons and holes to the extremely thin active region of such a DII laser. The energy levels of these electrons and holes are quantized, much that same as the classical “particle in a box”. This is shown schematically in fig. 6 for the case of a single GaAs quantum well bounded on each side by a Al~Ga1 ~As confining
turn-well heterostructure (QWII) lasers having active regions as thin as d 200 A have been grown by MOCVD [ill. Laser structures having coupled multiple-quantumwell active regions separated by thin (‘--‘10 100 A) higher-bandgap A1GaAs barrier layers can also be grown by MOCVD. Studies of such QWH opticallypumped and injection lasers have established that these structures exhibit a variety of unique and potentially useful optical and electrical properties [10 14,33 46]. The growth of such multiple thin-layer laser struc-
RD. Dupuis/A1GaAs
GaAs lasers grown by MOCVD
tures requires a materials technology that is capable of producing high-quality GaAs and Al~Ga1 ~As
Energy (eV) 1.45 I
epitaxial layers as well as abrupt Al~Ga1~As GaAs heterojunctions having alow densityof defects. Analysis of single- and multiple-layer MOCVD quantum-well heterostructures by Auger electron spectroscopy and simultaneous argon.ion sputter etching has established that the chemical-interface width of MOCVD Al~Gam ~As GaAs heterojunction is ~ 20A [7,47]. In particular, analysis of an Al045Ga055As GaAs multiple-quantum-well structure grown by MOCVD showed that the chemical interface width was ~ 17 A
219
1.50 1.55 1.60 I Al~Gai ~As GaAs (1+2)
L~ bOA t
311pm
300 K .~
[471. The high quality of MOCVD A1GaAs GaAs quantum-well heterostructures is evidenced by the fact that pulsed and CW room-temperature laser operation of optically-pumped and injection single- [10,11] and multiple- [33,37] quantum-well lasers grown by MOCVD have been reported. The low thresholds and high differential quantum efficiencies measured for
MOCVD ting these todevices note QWH further that materials the substantiate first [14].CW Inthe room-temperature fact, high itlasers quality is interesof grown singleThe by and 300 MOCVD rimultiple-quantum-well K emission [39,45]. spectra (pulsed diode and CW) of were an optically grown has by MOCVD ispumped shown quantum-well in fig. 7 [13]. structure This structure two GaAs quantum wells having a thickness L Z ““ 100 A. The curve (a) shows CW laser operation on the n = 1 1’ confined-particle transitions at an mcident power of ‘-—iü~W/cm2 - At higher pulsed excitation levels (b), laser operation occurs on both the n = 1 1’ and n = 2 2’ transitions [13]. Ouantumwell injection lasers have also operated CW on con-
fined-particle transitions at energies above the GaAs bandedge [391. The properties of QWH lasers are unique in many significant respects. Typically, MOCVD QWH A1GaAs GaAs lasers exhibit low thresholds, high-energy spectral emission (relative to bulk GaAs), and high external differential quantum efficiencies [37,39,45] other desirable features of diode lasers having QWH
active regions are that such devices tend to operate in a single longitudinal mode [451Multiple-quantum-well DH laser structures grown by MOCVD exhibit yet another interesting feature a strong coupling of LU phonons to the large con-
~ .~
—
I fl
.~ u 8.8— Jbm2(CW) ~ a) I I I 1 8.41’
I
I
I
8.0 I 2 2’ 7.6 3A) Wavelength Fig. 7. Emission spectra (300 K)(1001 an optically-pumped MOCVD multiple-quantum-well laser. The calculated confined-particle quantum-well transition energies are shown as dark (n, e —~hh) and open (n’, e —~ lh) bars along the wavelength axis. Laser operation occurs on confinedparticle states for both CW and pulsed excitation.
fined-particle electron population in the conduction band of such QWH lasers [41 46]. This coupling
leads to optical emission at energies corresponding to a quantum-well confined electron hole state shifted down in energy by an amount equal to one or more LU phonons. In fact, laser operation of optically-pumped multiple-quantum-well heterostructures has been achieved on phonon-assisted confinedparticle recombination transitions at energies as much as four LU Phonons from the corresponding
confined-particle
energy
level
[14,44,45,49,50].
Thus phonon interaction (principally with LO phonons) leads to profound changes in the laser spectra of such structures.
220
R.D. Dupuis/AIGaAs GaAs lasers grown by MOCVD
The MOCVD process has also been used to grow quantum-well heterostructures consisting of thin GaAs wells (Lz 50 A) and thin AlAs barriers (LB 50 A) [51,52]. Such structures exhibit a greatly reduced phonon coupling in their lasing characteristics owing to the elimination of alloy clustering in the pure AlAs barriers [50]. These results have shown that alloy clustering in Al~Ga 1 ~As, 0
5. MOCVD CW laser lifetimes
longitudinal mode for the entire length of the test (t > 3200 h). While these results were obtained on unoptimized devices, they indicate that injection lasers with useful lifetimes can be grown by MOCVD. Further detailed studies are needed to identify any defects in MOCVD materials that might limit laser lifetimes.
6. Summary and conclusions Metalorganic chemical vapor deposition has been shown to be a viable materials technology for the growth of a wide variety of A1GaAs GaAs laser structures. The performance characteristics of MOCVD conventional DII lasers having both GaAs and A1GaAs active regins have been shown to be equal to or better than the characteristics of similar DH laser structures grown by liquid-phase epitaxy and molecular-beani epitaxy. In addition, the MOCVD process has been shown to be useful in the growth of novel DII laser structures that exhibit room-temperature CW single-mode lasers having linear light-output versus current characteristics. High-quality single- and multiple-quantum-well Dl-1 laser structures have also been grown. These structures consist of epitaxial layers as thin as —-‘10 A and clearly demonstrate the capability of the MOCVD process to grow ultra-thin layers. Furthermnore, the lasing characteristics of these structures
imply that the MOCVD A1GaAs GaAs heterojuncIt is of great practical interest to determine the CW lifetimes of MOCVD injection lasers. Lifetimes ~ i0~ h at 300 K are generally required for practical laser applications. While only preliminary data on the lifetime of MOCVD lasers has been published [20 22], the performance of these devices has been exceedingly promising as mentioned above. For example, constant-current continuous operation at roomtemperature (“--‘26°C) has been achieved for CW stripe-geometry mnultiple-quantum-well lasers for times greater than 3200 h [22]. The diodes of this study had an “active region” consisting of six GaAs quantum wells with a thickness “—‘120 A separated by five Al0 30Ga0 70As barriers also “-‘120 A thick. No significant degradation in the output of one of these devices was observed during the first —‘700 h of CW operation. In addition, this device lased in a single
tions are extremely uniform and abrupt and contain to large interface non-radiative recombination centers.
The promising (although preliminary) data on CW 300 K MOCVD lasers published to date indicates that useful DH lasers can be made by MOCVD and that there are no fundamental barriers to the future development of long-lived MOCVD lasers. These results for MOCVD AlGaAs GaAs layers, and the inherent advantages that the MOCVD process offers in the growth of large area, uniform epitaxial layers indicate that metalorganic chemical vapor deposition is a very promising technology for the large-scale growth of AlGaAs GaAs lasers and other heterojunction devices.
RD. Dupuis
/ AIGaAs
References [1] ll.M. Manasevit, Appl. Phys. Letters 12 (1968) 156. [2] I-or a review, see: H.M. Manasevit, J. Crystal Growth 13/14 (1972) 306; H.M. Manasevit, J. Crystal Growth 22 (1974) 125. [31 RD. Dupuis and P.D. Dapkus, Appl. Phys. Letters 31 (1977) 466. [4] R.D. Dupuis and P.D. Dapkus, IEEE J. Quantum Electron. QL-15 (1979) 128. [5] S.D. Hersee, J.P. Hirtz, Bui-Dinh-Vuong, J.P. Duchemi M. Bonnet, G. Mesquida and A. Parent, 7th IEEE Intern. Semiconductor Laser Conf., Brighton, UK, 1980. [6] RD. Dupuis and PD. Dapkus, Appl. Phys. Letters 32 (1978) 473. [7] R.D. Dupuis, L.A. Moudy and PD. Dapkus, in: Proc. 7th Intern. Symp. on GaAs and Related Compounds, St. Louis, 1978, Inst. Ph~s. Cont. Ser. 45 (Inst. Phys., London, 1979) ~ 1. [8] E.J. Thrush, P.R. Selway and GD. Henshall, Electron. Letters 15 (1979) 156. [9] H. Kressel and M. Ettenberg, J. AppI. Phys. 47 (1976)
3533. [10] R.M. Kolbas, N. Holonyak, Jr., R.D. Dupuis and P.D. Dapkus, Pisma Zh. Tekh. Hz. 4 (1978) 69 [Soviet Tech. Phys. Letters 4 (1978) 28]. [11] RD. Dupuis, P.D. Dapkus, N. Unlonyak, ir., L.A. Rezek and R. Chin, Appl. Phys. Letters 32(1978) 295. [12] N. Holonyak, Jr., R.M. Kolbas, W.D. Laidig, B.A. Vojak, RD. Dupuis and P.D. Dapkus, in: Proc. 7th Intern. Symp. on GaAs and Related Compounds, St. Louis, 1978, Inst. Phys. Cont. Ser. 45 (Inst. Phys., London, 1979) p. 387. [13] RD. Dupuis, PD. Dapkus, R.M. Kolbas and N. Holonyak, Jr., IFFE i. Quantum Electron. QE-15 (1979) 756. [14] N. Holonyak, Jr., R.M. Kolbas, R.D. Dupuis and PD. Dapkus, IFLE J. Quantum Electron. QE-16 (1980) 170. [15] For a discussion of quantum-size effects in III V semiconductor heterostructures, see R. Dingle, in: I estkorperprobleme XV, Advances in Solid State Physics, Ed. Iii. Qucisser (Pergamon Vieweg, Braun21. sLhweig, Germany, 1975) p. (16) A discussion of non-equilibrium carrier populations in quantum-well structures is given by K. Hess and N. Holonyak, ir., Phys. Today 33 (1980) 40. [17] W.T. Tsang, Appl. Phys. Letters 36(1980)11. [18] R.D. Dupuis and PD. Dapkus, Appl. Phys. Letters 31 (1977) 839. [19] RD. Dupuis and PD. Dapkus, Appl. Phys. Letters 32 (1978) 406. 120] E.J. Thrush and J.E.A. Whiteaway, Electron. Letters 15 (1979) 666. [21] Y. Mon and N. Watanabe, Electron. Letters 16(1980) 285. [22) R.D. Dupuis, AppI. Phys. Letters 35 (1979) 311.
GaAs lasers grown by MOCVD
221
[23] T.L. Paoh and i.E. Ripper, Appl. Phys. Letters 25 (1974) 744. [24] For a discussion of spectral narrowing of homogeneously broadened transitions, see: W.V. Smith and P. D. Sorokin, in: The Laser (McGraw-Hill, New York, 1966). [25] RD. Dupuis and P.D. Dapkus, AppI. Phys. Letters 33 (1978) 724. [26] J.J. Coleman and P.D. Dapkus, Appl. Phys. Letters 37 (1980) 262. [27] Y. Mori, 0. Matsuda, K. Morizane and N. Watanabe, Electron. Letters 16(1980). [28] D. Fekete, RD. Burnham, DR. Scrifres, W. Streifer and RD. Yinghing, Appl. Phys. Letters 38 (1981) 607. [29] N. Nishi, M. Yan”, Y. Nishitani, Y. Akita and M. Takusagawa, Appl. Phys. Letters 35 (1979) 232. [30] II. Veenvliet, C. van Opdorp, R.P. Tijburg and J.P. Andre, IEEE J. Quantum Flectron. QE-l5 (1979) 762. (31] J.P. Noad, CM. Look and A.J. Springthorpe, Llectron. Letters 16 (1980) 685. [32] R. Dingle, W. Wiegmann and CU. Henry, Phys. Rev. Letters 33(1974) 827. [33] N. Holonyak, Jr., R.M. Kolbas, RD. Dupuis and P.D. Dapkus, Appl. Phys. Letters 33 (1978) 73. [34] R.D. Dupuis, PD. Dapkus, R.M. Kolbas and N. Holonyak, Jr., Solid State Commun. 27 (1978) 531. [35] R.D. Dupuis, P.D. Dapkus, R.M. Kolbas, N. Holonyak, Jr. and H. Shichijo, AppI. Phys. Letters 33 (1978) 596. [36] 1-1. Shichijo, R.M. Kolbas, N. Flolonyak, Jr., RD. Dupuis and PD. Dapkus, Solid State Cominun. 27 (1978) 1029. [37] N. Holonyak, Jr., R.M. Kolbas, W.D. Laidig, BA. Vojak, RD. Dupuis and P.D. Dapkus, i. Appl. Phys. Letters 33(1978) 737. [38] N. Holonyak, Jr., R.M. Kolbas, E.A. Rezek, R. Chin, RD. Dupuis and PD. Dapkus, J. Appl. Phys. 49 (1978) 5392. [39) RD. Dupuis, PD. Dapkus, R. Chin, N. Flolonyak, Jr. and SW. Kirchoefer, Appl. Phys. Letters 34 (1979) 265. [40] RD. Dupuis, P.D. Dapkus, N. Ilolonyak, Jr., R.M. Kolbas, W.D. Laidig and BA. Vojak, Pisma Zh. Tekh. Fiz. 5 (1979) 132 [Soviet Tech. Phys. Letters 5 (1979) 52]. [41] N. Holonyak, Jr., R.M. Kolbas, W.D. Laidig,M. Altareili, RD. Dupuis and PD. Dapkus, Appl. Phys. Letters 34 (1979) 501. [42] B.A. Vojak, SW. Kirchoefer, N. Holonyak, Jr., R. Chin, R.D. Dupuis and P.D. Dapkus, J. AppI. Phys. 51) (1979) 5810 [431 B.A. Vojak, N. Holonyak, Jr., R. Chin, E.A. Rezek, R.D. Dupuis and P.D. Dapkus,J. AppI. Phys. 50 (1979) 5835. [44] R.M. Kolbas, N. Holonyak, Jr., B.A. Vojak, K. Hess,
S SI
SI
55 55 S5 S5 55 V V V
222
[45] [46]
[47] [48] [49]
R.D. Dupuis /AlGaAs GaAs lasers grown by MOCVD M. Altarelli, R. D. Dupuis and P.D. Dapkus, Solid State Commun. 31(1979)1033. RD. Dupuis, P.D. Dapkus, N. Holonyak, Jr. and R.M. Kolbas, App!. I’hys. Letters 35 ~l979j 487. N. Holonyak, Jr., R.M. Kolbas, W.D. Laidig, BA. Vojak, K. Hess, R.D. Dupuis and P.D. Dapkus, J. AppI. Phys. 51(1980)1328. R.D. Dupuis, PD. Dapkus, C.M. Garner, C.Y. Su and W.E. Spicer, Appl. Phys. Letters 34 (1979) 338. M. Ettenberg, Ci. Nuese and H. Kressel, J. Appl. Plsys. 50(1979)2949. K. Hess, Appl. Phys. Letters 35 (1979) 484.
[50] B.A. Vojak, N. Holonyak, Jr., W.D. Laidig, K. Hess, J. J. Coleman and P.D. Dapkus, J. App!. Phys. 52 (1981) 959. [51] N. Holonyak, Jr., W.D. Laidig, B.A. Vojak, K. Hess, ii. Coleman, P.D. Dapkus and J. Bardeen, Phys. Rev. Letters 45 (1980) 1703. [52] J.J. Colensan, PD. Dapkus, W.D. Laidig, B.A. Vojak and N. Holonyak, Jr., Appl. Phys. Letters 38 (1981) 63. [53] B.A. Vojak, W.D. Laidig, N. Holonyak, Jr. M.D. Camras, J.J. Coleman and P.D. Dapkus, J. AppI. Phys. 52 (1981)621.
S