Tunable absorption and electroluminescence in GaAs doping superlattices

Superlattices and Microstructures, Vol. 3, No. 3, 1987

277

TUNABLE ABSORPTIONAND ELECTROLUMINESCENCE IN GaAs DOPING SUPERLATTICES G. Hasnain, C.J. Chang-Hasnain, G.H. DShler a, J . N . M i l l e r a, N.M. Johnson b, J.R. Whinnery and A. Dienes E l e c t r i c a l Engineering Department and Electronics Research Laboratory University of C a l i f o r n i a , Berkeley, C a l i f o r n i a 94720 aHewlett Packard Laboratories, 1501 Page M i l l Road, Palo Alto, C a l i f o r n i a bxerox Research Center, 3333 Coyote H i l l Road, Palo Alto, C a l i f o r n i a Received September I0, 1986

Highly tunable electroluminescence is observed in GaAs doping superlattice (n-i-p-i crystal) at room temperature with peak energies shifted more than 600 meV below the bulk bandgap (X > 1.55 um). Peak efficiency is about 2 %. Tunability of the optical absorption spectrum with p-n junction bias is also demonstrated by both photoconductivity and direct transmission measurements. A change of transmission of about 9% is obtained at 0.89 um wavelength through a 1.95 um thick n - i - p - i crystal by varying the p-n junction bias between -0.5 V and 0.5V.

Introduction The basic e l e c t r o n i c properties of a doping s u p e r l a t t i c e ( n - i - p - i c r y s t a l ) are tunable as a consequence of the s p a t i a l l y i n d i r e c t bandgap which can be varied with optical or e l e c t r i c a l e x c i t a t i o n [1,2]. The absorption c o e f f i c i e n t of a semiconductor at photon energies below the bandgap increases in the presence of an e l e c t r i c f i e l d due to the Franz-Keldysh e f f e c t [31. In a n i p-i c r y s t a l , a large e l e c t r i c field is built in due to the ionized impurities. Hence, by varying the e f f e c t i v e bandgap which varies the e l e c t r i c f i e l d , the absorption c o e f f i c i e n t can be tuned. The luminescence obtained by radiative recombination of excess electrons and holes across the s p a t i a l l y i n d i r e c t bandgap can s i m i l a r l y be tuned by changing the e f f e c t i v e bandgap. Several experiments have already demonstrated the t u n a b i l i t y of luminescence [4,5] and absorption [6,7] using optical excitation. However, for practical device applications such as modulators and tunable emitters i t is necessary to obtain e l e c t r i c a l tunability. This is achieved by applying s e l e c t i v e contacts [8] to permit e f f i c i e n t i n j e c t i o n of electrons and holes into the nand p- layers respectively by an external bias voltage. The e f f e c t i v e bandgap of the n - i - p - i is then equal to the separation of the quasi Fermi l e v e l s . This separation, ~|l ~p, corresponds to the external p o t e n t i a l eUnp applied between the s e l e c t i v e contacts, i f the

0749-6036/87/030277 + 06 $02.00/0

gradient of Cn and Cp is small enough to be neglected. When a small forward or reverse bias is applied to the contacts, the separation of the quasi Fermi levels is changed which changes the built-in electric field. Consequently, the f i e l d assisted absorption at photon energies below the host bandgap can be g r e a t l y changed with small change of the applied bias voltage. When a strong forward bias is applied to these contacts, the injected electrons and holes, though spatially separated, can radiatively recombine by tunneling through the potential barrier r e s u l t i n g in electroluminescence (EL) whose peak photon energy is ~w = ~n " ~p ~ eUnp

(])

Tunable Absorption Previous measurements of tunable absorption in a n - i - p - i crystal used optical e x c i t a t i o n [6,7] to vary the absorption c o e f f i c i e n t . For a practical modulator i t is necessary to vary the optical absorption using e x t e r n a l l y applied voltages which also permit reverse as well as forward bias. By using highly s e l e c t i v e contacts to the n- and p- layers, we have for the f i r s t time observed the tunable absorption with various e l e c t r i c a l bias (forward and reverse) using both photoconductivity and transmission measurements. The sample was grown by molecular beam epitaxy (MBE) using the recently reported shadow-growth technique [8] on undoped GaAs © 1987 Academic Press I nc, (London) Limited

Superlattices and Microstructures, Vol. 3, No. 3, 1987

Vnn

278

_ _ _ n-layers II

p-layers

VPn

~

p-contacts

~

n-contacts

Inn

J

ln-i-.-,SUBSTRATE In-i-p-il.-,-p-,l

/

~"I

/

Fig. i. Schematic of the n - i - p - i device used for both transmission and photoconductivity measurements.

WAVELENGTH (nm) 800

I--

Z

I,LI I--

104

>-

103

a. --

102

850

900

950

i

i

i

1000 i

Upn -0.35 -0.25 0 +0.1

N ~ l°1 _3

+0.2 +0.3

o

~& 100 1.55

1

I

I

I

I

I

1.50

1.45

1.40

1.35

1.30

1.25

ENERGY (eV) Fig. 2. Absorption spectra ~(~) of GaAs n - i - p - i BI060 at 300 K for various pn junction bias Unp from -0.35 V to 0.3 V. o(~) is obtained by measuring the photoconductivity

spectra through the p-layers with various bias = nA = 1018 cm-3 ; di = dp = voltage Un 4~Dnm. 50 nm ;

substrate. I t i~ds 41 layers of a l t e r n a t i n g GaAs n, i, p, i layers oF thickness 40, 50, 50, 50 nm respectively. The doping density is 1018 cm-3for both n and p layers. Using standard photolithography two pairs of n- and pcontacts (Figure I) are applied to the n - i - n - i and p - i - p - i regions respectively. The region in between l i k e - p a i r s of contacts are etched so that when a small voltage Upp is applied

between the two p- contacts~ the current Ipp is proportional to the conductance of only'-the p layers of the n - i - p - i . Similarly, Inn r e f l e c t s only the n-layer conductance. When the n - i - p - i is illuminated with a probe beam, the change of p-layer current due to photoconductivity is given by ~Ipp which is proportional to the two dimensional hole density generated by the probe beam, and

dnP-

Superlattices and Microstructures, VoL 3. No. 3, 1987 0.10

~ 0.08

i

f

I

I

279

I

i

I

\

i

i

i

-

Vl=-O.5V

0 @rJ 0.06

a~ U,I¢.O N z

oo.

zoO 0.02 0.0

I

I

i

i

I

~!

m III

~

• IlllWlm '

900

-

w"

950

.. "

~ h ~ L -

p.

-

•

.AI

-

-

.. --

w

WAVELENGTH (nm) Fig 3. Modulation of optical transmission through GaAs n-i-p-i BI060 by applied pn junction bias Unp.

therefore to the absorption coefficient a(~). A voltage Unp is applied between the n- and pcontacts in order to forward or reverse bias the p-n junctions and thus vary the b u i l t - i n electric field. Figure 2 shows the absorption spectra with various p-n junction b i a s (Unp) obtained by photoconductivity measurements. AXe arc lamp and a spectrometer was used to generate a tunable monochromatic probe beam. At photon energy 180 meV below the bulk bandgap, the absorption coefficient is varied by a factor of 30 with only 0.65 V bias. Although absolute values of the absorption coefficient can not be obtained by this measurement, a(w) at photon energies much higher than GaAs bandgap can be approximated by that of the bulk material with same doping density (independentof Unp). The normalized change of transmission is obtained by measuring the transmission coefficient with various Uno biases (Figure 3). The probe beam was focuss6d on the device in the direction perpendicular to the layers. A change of transmission of 9% is observed at 0.89 um by varying Unp only between -0.5 V and 0.5 V. Quantitative comparison between the two measurements is d i f f i c u l t due to the lack of the absolute values of a(~). However, the change of transmission obtained from the two measurements agree within a factor of 2. Tunable Electroluminescence

Earlier EL measurements were made with alloyed selective contacts [g] but these contacts develop h i g h leakage currents and become unselective at high doping

concentrations (> I018cm-3) needed for room temperature tunability. Using the shadow growth technique [8] highly selective lateral contacts are now made feasible even at very high doping. T h i s has resulted in very low contact resistance and excellent p-n diode characteristics. Consequently, efficient lateral injection EL has been obtained even at room temperature. The tunable EL spectra of a GaAs n-i-p-i sample with 1018 cm-3 doping concentration is shown in Figure 4 at different temperatures from 4K to 30OK. The corresponding applied bias voltage Upn is shown alongside each spectrum. This is the f i r s t observation of EL under such low applied voltage indicating near perfect contacts. Previous EL measurements, [10,11] even at 4K, required several volts of applied bias to o b s e r v e appreciable lum]nescence since much of the potential drop occurred at the contacts and much of the current was due to non-radiative recombination at the poor contacts. Much larger tunability is now observed due to improved luminescence efficiency. At low temperatures (< 77K) EL with peak photon energies down to 1.22 eV, or, in other words, a total tunability of nearly 300 meV is observed. Since the lateral dimensions of the n-i-p-i were chosen quite large Corder of mm) to f a c i l i t a t e fabrication, voltages greater than the effective bandgap are s t i l l required at higher injection levels because of potential drop in the series resistance of the n-i-p-i layers. The origin of the double-peak (or dip-like) structure observed at higher temperatures n e a r the (temperature dependent) bulk bandgap is not yet clear.

Superlattices and Microstructures, Vol. 3, No 3, 198-/

280

I-

i,(

1.2

1.4

1,2

1,6

4K

77K

B1014

B1014

1.4

PHOTON ENERGY (eV) 1.2 1.4 1.6

1.6

~,

150K B1014

3.72V

3.72V

IO Z

3.21W

2.3SV

2.35V

1.7'2V

1.'P~V ~:~:-,/+. '.# :,=

BI

2.36V :I.NV 1.7"2V

II1

1.2

1.4

'225K 111014

1.2

1+6

+~

1.6

B1014

3.26V

\~ ' ~

::_:

4.0V

_-

~,~

2.35V

/+v '/!+:

1.7GV

'+'",."L:=

2.341V --I+ ~ 1.44V

' ~-~

I .~v~~,.~.

1.35 A _ _ : _ -'- - - _\

W

1+4

+

1

,

2

~

z

,.-v

__

I'-" U

I .lV

1. ~ V

I .nV ,

1.1

1.o

o.g

0.8

1.1

1.o

o.9

o.I

1.1

w

1.o

o.g

o.I

,

,

1.1

,

,

1.o

,

o.9

,

0.8

I

1.1

1.o

0.9

0.8

WAVELENGTH(~m) 1018 c m 3 ; periods.

Fig. 4. EL spectra at d i f f e r e n t temperatures from 4K to 300K of GaAs n - i - p - i B1014 f o r d i f f e r e n t a p p l i e d bias v o l t a g e s , nD = nA

(n I.-.

0.8

1.0

1.21.4 ,

4.2K B1054

,

,

,

ENERGY (eV) 1.0 1 . 2 1 . 4

0.8 ,

,

,

•

+

,

r

dn

0.8

40

nm;

dp

bO nm;

:10.5

1.0 1.2 1.4

,

77K B1054

CO .,9[

.v

4V

N

z

w F-

,v

_z

2.5V

~.ov _~-~//\/~{

(..tl

1.§v

z

1 .SV

,..v-~

SI-

l.~1

w

z

1.21v

_

'-°'-v-~L \ ~ /

m (.)

w

M .i

I ,lOV

1:6 ' 1:4~1.'2 ' 1.0 'o.al.el:4 ' + '

£

i

£

~1.'2 1:0'0:sl.61.4 WAVELENGTH (.m)

dn

~

i

i

&

i

, I

1.2 1.0 0.6

Fig. 5. EL spectra at 4K, 77K and 300K of GaAs n-i-p-i BI054 having higher doping for d i f f e r e n t a p p l i e d bias v o l t a g e s , nD = nA = 4 X

1018 cm-3; periods.

= 30

nm;

dp

= 35

um;

10.5

At a f i n i t e temperature, the e l e c t r o n s and holes can be t h e r m a l l y e x c i t e d to the top of t h e ~ p o t e n t i a l b a r r i e r and recombine v e r t i c a l l y to emit photons of energy equal to the bulk bandgap. Therefore, the t u n a b i l i t y g r a d u a l l y

degrades at higher temperatures and the spectra broaden as thermally assisted tunneling recombinations i n v o l v i n g higher subbands begin to participate and eventually thermally s t i m u l a t e d v e r t i c a l recombinations dominate at

Superlattices and Microstructures, Vol. 3, No. 3, 1987 room temperature as expected f o r the doping level used. It has been t h e o r e t i c a l l y suggested [i[2] and e x p e r i m e n t a l l y v e r i f i e d by PL measurements [4,5] that f o r s u f f i c i e n t l y high doping concentration ( nD, nA > 3 X i0 I~B cm-3 for GaAs) the tunable tunneling recombination w i l l dominate at room temperature over the ph,n,)u assisted d i r e c t recombination. Tunabl~ elect roluminescence is thus obtained at room temperature as shown in Figure 5 using a GaAs n - i - p - i with higher doping: nD = nA = 4 X 1018 cm-3. Over the temperature range from 4K to 300K the EL spectra peak energy is observed to be tunable down to 1.0 eV corresponding to a 400 meV s h i f t at 300K and a 500 meV s h i f t below 77K. The observed EL t u n a b i l i t y compares very well with theory and with that measured by PL [4,5] from a very s i m i l a r n-i p - i . At 77K and lower temperatures when the phonon assisted recombination is n e g l i g i b l e , the EL peak is observed e x a c t l y at the applied bias times the elementary charge, thus v e r i f y i n g equation (1) f o r the f i r s t time. Again, this exact correspondence is not observed at high i n j e c t i o n currents due to the series pot,~ntial drop in the l a t e r a l l y wide layers. The luminescence e f f i c i e n c y of t h i s n - i - p - i is comparable to that of a commercial double -heterosl: ruct ure (DH) LED. The peak external e f f i c i e n c y of the n - i - p - i was nearly 2%. This GaA,_. n - i - p - i also had ] ~tm t h i c k AIo.3Gao.7A,_. cladding layers to provide o p t i c a l confinement in the v e r t i c a l d i r e c t i o n . The s p { ( t r a shown in Figure 5 were obtained from tl,~ EL emil:ted l o n g i t u d i n a l l y from one of the cleaved facets. At 4K we observe dramatic narrowing uf the spectrum owing to onset of stimulated enlission. This device, not optim]sed at a l l f o r ]asi~ig, was observed to lase with a threshold current of 25 mA. At such high inLiection level the s u p e r l a t t i c e space charge p o t e n t i a l variation is almost f l a t t e r e d and the quasi Fermi level separation i,.. neariy equal to the bulk bandgap. Hence the lasing wavelength (>, - 834.76 nm) nearly corresponded to the bulk bandgap. However, we expect that with optimised device geometry and improved n i-p i design we w i l l be able to reduce the threshold s u f f i c i e n t l y to observe lasing well below the bu]k bandgap, and possibly at room temperature. Figure 6 depicts a first achievement towards t h i s goal. By increasing the p-doping level to nA "~ ]0 ]9 cm'3, the p - l a y e r series re~i~tance i~ strongly reduced and the transistion probabilities for tunne]ing recombinations are s i g n i f i c a n t l y enhanced. The former r e s u l t s in a r a t h e r uniform EL i n t e n s i t y p r o f i l e over the whole n i p-i region compared to previou~ observations [8] where the EL i n t e n s i l y was concentrated near the p contact. The l a t t e r ~,ields a strong red s h i f t of the EL spectra for the given range of current densities. More inlportant, the o p t i c a l power emitted fa~ below the bulk bandgap (more than 600 me't) is correspondingly increased by

281

ENERGY (eV) 0.7

0.9

1.1

1.3

1.5

(n c

k,,

,< v

UJ 0 Z U,I (b (n

uJ z i

5 .-I

0mr" I-(J UJ ..J

LU

1,8

1.6

1.4

1,2

1.0

0.8

WAVELENGTH (~m) Fig. 6. EL spectra at 300K of GaAs rl i F i B1167 with increased p doping f o r d i f f e r e n t applied bias voltages, nD = 4 X 10 ]8 cm 3: rlA = 1019 cm-3; d n = 25 nm; dp = 35 nm; [0.5 periods.

several orders of magnitude. For p o t e n t i a l use in f i b e r - o p t i c communications, we note that with 0.66V bias, the GaAs n i - p - i is observed to emit l i g h t at 1.58 um. Conclusions In conclusion, we have demonstrated f o r the first time highly tunable and e f f i c i e n t electroluminescence at room temperature thus proving the f e a s i b i l i t y of a tunable n i - p - i LED. We also demonstrated the first v e r i f i c a t i o n of the large t u n a b i l i t y of the o p t i c a l absorption spectra with very small modulating voltage. The r e s u l t s i n d i c a t e good possibility of achieving a tunable n i - p - i l a s e r and a h i g h l y s e n s i t i v e n - i - p i modulator in the near f u t u r e . Acknowledgement The authors thank 1. Low, G. T r o t t , T.R. Ranganath, S.Y. Wang and D. Bimberg for helpful discussions, D. Mar~ and A.

Superlattices and Microstructures, Vol. 3, No. 3, 1987

282 Fischer-Colbrie for growing the samples, and R. Lacoste and B. Boatman for technical assistance. This work was p a r t i a l l y supported by a grant from the National Science Foundation under contract number ECS-8114526.

References [ I ] G.H.D6hler, Phys. Star. Sol. (b) 52, 79 and 533 (1972); J. Vac. Sci. Technol. 16, 851 (1979) [2] For a review see: K. Ploog and G.H. Dohler, Adv. Phys. 32, 285 (1983) [3] L.V. Keldysh, Sov. Phys. JETP 7, 788 (1958) [4] G . H . D6hler, G. Fasol, T.S. L o w , J.N. M i l l e r and K. Ploog, Solid State Commun. 43, 563 (1986)

[5J K. K6hler, G.H. D6hler, J.N. Miller and K. Ploog, Solid State Commun. 58, 769 (1986) [6] G.H. Dohler, H. Kunzel and K. Ploog, Phys. Rev. B 25, 2616 (1982) [7] T.B. Simpson, C.A. Pennise, B.E. Gordon, J.E. Anthony and T.R. AuCoin, International Quantum Electronics Conference, San Francisco, June 1986 [8] G.H. D6hler, G. Hasnain and J.N. M i l l e r , Appl. Phys. heLL. 4£~12), 704 (1986) [9] K. Ploog, H. K~nzel, J. Knecht, A. Fischer and G.H. D6hler, Appl. Phys. Lett. 38(]I~, 870 (1981) []0] H. Jung, H. K~nzel, G.H. D6hler and K. Ploog, J. Appl. Phys. 54(12~, 6965 (1983) [11] G. A b s t r e i t e r , Two-dimensional systems Heterostructures and Superlattices, Proc. Intl. Winter School, Mauterndorf (Springer-Verlag, Berlin, 1984) [12] G.H. D6hler, J. Vac. Sci. Technol. B, 1,2~, 278 (1983)