Efficient GaAsGa1−xAlxAs heterostructure electroluminescent diodes

SolidSStnteElectronics

Pergamon

Press 1971. Vol. 14, pp. 1265-1273.

Printed in Great Britain

EFFICIENT GaAs-Ga,,Al,As HETEROSTRUCTURE ELECTROLUMINESCENT DIODES E. A. ULMRR, JR.* Bell Telephone Laboratories, Murray Hill, New Jersey 07974, USA. (Received 16 April 197 1; in revisedform 14 June 197 1) Abstract-The development of efficient GaAs(Zn) electroluminescent diodes using a GaAsGa,_,Al,As single heterostructure design is reported. External equantum efficiencies (300°K) of 10 per cent have been achieved at 9100 A (1.36 eV) with pulsed current densities at and above 70 A/cm* on square diodes embedded in epoxy domes. The heterostructure consists of a Ga,_,Al,As (Zn) p-layer grown by liquid phase epitaxy on an n-type GaAs substrate with the simultaneous diffusion of Zn a distance of - 2 pm into the substrate. Several features of the heterostructure design contribute to the high efficiency: (1) the 9100 di emission suffers little absorption in the Ga,_,Al,As, (2) there is little nonradiative recombination at the GaAs-Ga,_,Al,As interface, and (3) the compensated p-region produces 9 100 A radiation which is not strongly absorbed in the n-GaAs regions of the device. The external quantum efficiencies obtained with the heterostructure devices are nearly an order of magnitude higher than those obtained from conventional Zn-diffised GaAs homostructure diodes with similar geometry. The solution growth, fabrication, and electroluminescence properties of the heterostructure diodes are described. R&sun&-On presente un rapport sur le developpement de diodes 6lectrohuninescentes efficaces en GaAs(Zn) en employant une conception d’Wt&ostructure simple au GaAs-GaI_,AlzAs. Des efficacites quantiques (300°K) de 10% ont CtCobtenues a 91OOA (1,36 eV) avec des dens& de courant pulse &gales et superieures B 70 A/cm2 sur des diodes car&es encapsul6es dans des domes en tpoxide. L’heterostructure consiste d’une couche p de Ga,_,Al=As(Zn) cultivCe par Cpitaxie en phase liquide sur une base au GaAs du type n avec la diffusion simultanee de Zn jusqu’a environ 2 pm dans la base. Plusieurs caracdristiques de la conception de I’hCt&ostructure contribuent B la grande efficacite: (1) l’emission de 9100 8, est peu absorb&e dans le Ga,_,Al,As; (2) il existe peu de recombinaison non radiative a I’interface GaAs-Ga,_,Al,As et (3) la region p compensee produit une radiation de 9 100 A qui n’est pas fortement absorbee dans les r&ions n-GaAs du dispositif. Les efficacites quantiques extemes obtenues avec les dispositifs B hCt6rostructure sont p&s d’un ordre de grandeur plus tlevees que celles obtenues de diodes B heterostructure classiques GaAs au Zn diffuse de gtomttrie semblable. On d&tit la culture de la solution, la fabrication et les propribtes d’tlectroluminescence des diodes 21 heterostructure. Zusammenfassung- Uber das Entwurfsprinzip fur zinkdotierte GaAs-Lumineszenzdioden mit einer einfachen GaAs-Ga,_,AI,As-Heterostruktur wind berichtet. Der exteme Quantenwirkungsgrad betriigt 10% bei 300°K und einer Wellenltige von 910 nm entsprechend I,30 eV. Dabei wurden die in eine Epoxidkuppel eingebetteten quadratischen Dioden gepulst betrieben bei Stromdichten von 70A/cm2 und dariiber. Die Heterostruktur besteht aus einer Zn-dotierten p-Schicht aus schmelzepitaktisch abgeschiedenem Ga,_,As,As auf einem n-Typ GaAs-Substrat mit einer gleichzeitig erfolgten, 2 @rn tief ins Substrat reichenden Zn-Diffusion. Einige Eigenschaften der Heterosttuktur tragen zu dem hohen Wirkungsgrad bei: (1) Die Emission bei 910 nm erfahrt in der Mischkristallschicht eine nur geringe Absorption, (2) an der Phasengrenze zwischen GaAs und Ga,_,Al.,.As tritt nur eine geringfiigige nichtstrahlende Rekombination auf und (3) das in der kompensierten p-Zone erzeugte Licht bei 9 10 nm wird such im n-Typ Bereich nur wenig absorbiert. Der beobachtete exteme Quantenwirkungsgrad ist bei gleicher Geometrie urn etwa eine GrijSenordnung hiiher als bei Zn-ditfundierten reinen GaAs-Dioden. Der ProzeS der Schmelzepitaxie, die Herstellungsvorgihrge und die Elektrolumineszenzeigenschaften der Heterodioden werden beschrieben. 1. INTRODUCTION

hold junction lasers[l, 21. The heterostructure consists of a Ga,_,Al,As(Zn) p-layer grown by liquid phase epitaxy on an n-type C&As substrate with the simultaneous diffusion of Zn a distance of - 2 pm into the substrate (see Fig. 1). Several *Present address: The Singer Co. Little Fails, N.J. 07424. features of these laser diodes also make them 1265 SINGLE heterostructure GaAs-Ga,_,AI,As diodes have previously been used to fabricate low thres-

1266

E. A. ULMER, JR.

attractive as efficient sources of incoherent radiation: (1) the 9 100 A emission from the p-GaAs region can exit through the wide-gap Ga,_,Al,As with essentially no internal reabsorption, (2) there is little nonradiative recombination at the GaAsGa,_,Al,As interface[l, 21, and (3) the compensated p-region produces radiation which experiences only moderate absorption in the n-GaAs. It has been shown[l] that simple mesa heterostructure diodes are more efficient than similarly doped homostructure diodes. In this paper we describe

Fig. 1. Energy band diagram of a forward-biased single heterostructure diode. Electrons injected into the GaAs p-region we confined by the potential barrier at the Ga,-,Al,As interface.

the fabrication and electroluminescence properties of GaAs-Ga,_,Al,As single heterostructure diodes with room temperature external quantum efficiencies as high as 10 per cent and continuous power outputs up to 65 mW at 9100 A. The structure and doping levels of these electroluminescent diodes are the same as those of the single heterostructure junction lasers [ 1,2]. A number of workers have studied homostructure electroluminescent diodes of GaAs. These homostructure devices include those formed by Zn-diffusion into an n-GaAs substrate, and those which are grown by liquid phase epitaxy with Si doping on both sides of the junction. For GaAs(Zn) diffused diodes[3, 41 at 300°K the external quantum efficiencies are less than 1 per cent. Internal reabsorption of light is a serious problem in these diodes[5]. For Si compensated GaAs diodes grown by liquid phase epitaxy[6-91 the emission is centered near 93OOA and the internal reabsorption of light is reduced. For the latter diodes room temperature external quantum efficiencies up to 10 per cent have been reported

on uncoated rectangular diodes[9]. The use of special hemispherical domes of GaAs [81 or domes of a high-index glass[9] can increase these efficiencies to 25 per cent[l] and 32 per cent[9]. In Zn-doped GaAs homostructure diodes with shallow junctions, nonradiative.recombination near the surface can lower the radiative efficiency. Fabricating units with deeper junctions to avoid these unwanted surface effects only increase the problem of internal reabsorption[5]. In the heterostructure diodes described here, there is essentially no nonradiative recombination of the injected carriers at the GaAs-Ga,_,Al,As interface, and, therefore the problem of nonradiative sugace recombination is avoided. Still another advantage of these efficient diodes is their simple rectangular geometry. They do not require a hemispherical dome of GaAs to achieve these efficiencies. Alferov et al.[ 101 recently suggested using various Ga-Al-As structures as efficient sources of spontaneous radiation. However, they did not report on the single heterostructure we have employed. In the next section, we describe the apparatus, materials and procedures employed to grow Ga,_,Al,As with a tipping technique. The use of a source crystal to saturate the melt with As just prior to growth is described. The following section covers the details of the fabrication of the individual diodes. In subsequent sections, the electroluminescence properties of these heterostructure diodes are presented. We conclude with a discussion of the high efficiencies exhibited by GaAs-Ga,_,Al,As heterostructure diodes. 2. LIQUID PHASE EPITAXY

The diodes are a single epitaxy structure grown in an open system under a dry hydrogen ambient. The pyrolytic carbon boat assembly shown in Fig. 2 holds the gallium charge, the GaAs substrate, and the source crystal. The slider covers the GaAs substrate at all times, thereby preventing excessive As loss from the substrate prior to growth. In agreement with previous results on GaAs lasers [ 111, the substrate proved to be the most important factor in determining the quality of the diodes. The substrates used in the present experiments are %-doped n-type GaAs cut with either (100) or (111) orientations. Table 1 lists the properties of the substrates used in this study. Undoped (n = lOI cmw3) polycrystalline GaAs was

GaAs-GaAlAs

ELECTROLUMINESCENT

DIODES

1267

Table 1. GaAs substrate material Etch

Dwinsrange

Ingot*

Orientation

(X lOI elec. cmMS)

pit density (X 1Wcm-*)

18L 263B 270 321 357 387 G5-89

100 111 100 111 100 100 111

0.9-l .4 1.3-4.0 3.0-3.3 3.5-3.7 2.0-2.8 2.7-4.4 3.2

2.6-3.5 7.6-7.9 4.2-4.6 6.4-10.0 3.8-0.8 4.6-5.0 5.0

Photoluminescent efficiency Not measured Not measured Very goodt Ratio: 8.3 front, 2.6 back* Not measured Ratio: 5.0 front, 3.9 back+ Not measured

*All these ingots are n-type %-doped single crystal GaAs. All the ingots except GS-89 are from Laser Diode Laboratories, Metuchen, NJ. G5-89 is from Monsanto, St. Louis, MO. +I. Hayashi, private communication. #These are the photoluminescence efficiency ratios used by Kressel, et al. (Ref. [ 111) to evaluate GaAs substrates. The seed end of the ingot is labeled front.

used for: (1) the supply of As in the melt, and (2) the source crystal for final saturation of the melt. It should be emphasized that most of the As in the melt is derived from small pieces of polycrystalline GaAs. The source crystal guarantees saturation of the gallium melt just before it is tipped onto the substrate. A few milligrams of 99.999 per cent metallic Zn is placed in the melt along with 1.5 g of Ga. This amount of Zn produces the 2 pm diffusion into the heavily doped substrate. With the aid of the Ga-Al-As phase diagram [ 121, the melt composition is selected to produce a saturated solution at our normal tipping temperature of - 104O”C, and an initial composition in the regrown layer of G%b,Ab,,As. Initial growth temperatures ranged between 1040°C and 1050°C. At ‘tip-on’ the substrate is a few degrees warmer than the melt because of a small horizontal temperature gradient in the furnace. As a result, OUARTZ

THERMOCOUPLE

some dissolution of the GaAs substrate occurs before the growth of the Ga,_,Al,As. It is thought that dissolution removes impurities on the substrate surface, leaving the GaAs-Ga,_,Al,As interface free of impurity defects. The standard cooling rate is 3”C/min, with a total growth time of 90 min. Growth is terminated at 800°C. Following a 1 hr anneal at 78O”C, the quartz system is pulled from the furnace and allowed to cool. Photoluminescence measurements (on an angle lapped sample), combined with the data of Casey and Panish[ 131, indicate an AlAs fraction, x, at the GaAs-Ga,_,Al,As interface of about 0.6. This fraction is not fixed throughout the layer, but decreases with distance toward the surface (see Fig. 1). For thicknesses greater than 50 pm, the composition is nearly pure GaAs. The net hole concentration in the diffused p-layer has been estimated to be - 3 X 1018 cmm3 on similar single heterostructures [ 11.

MELT

3. FABRICATION

growth, the Ga,_,Al,As is lapped and etched to a thickness of 50 pm to remove the optically absorbing GaAs top portion of the grown layer. A Cr-Au contact (covering about 10 per cent of the Ga,_,Al,As surface area) is then evaporated through a ‘dot’ or ‘H’ mask. After the p-contact is applied, the n-side of the wafer is lapped to produce an overall thickness of 100 pm, and a contact evaporated over the entire n-side of the wafer. Next, the wafers are usually scribed and broken into 15 mil square units, although some wafers are sawed. The individual diode (shown in Fig. 3) is After

SLlDER

WA8 SUBSTRATE

SOURCE CRYSTAL

FURNACE WALL

Fig. 2. The solution growth apparatus.

E. A. ULMER, JR.

1268

bonded n-side down to a transistor header. After mounting, the diode is etched with a 3 : 1: 1 H,SO, : H,O,: Hz0 etch to remove surface strains introduced in the dicing operating. For index-matching a dome of clear epoxy (n = 1.53) is placed over the diode.

Fig. 3. Sir&e-heterostructure electroluminescent diode. 4. RESULTS (a) External eficiency The external quantum efficiency was measured in a calibrated integrating sphere equipped with two detectors: a 2 cm2 Si solar cell, and a calibrated

Si photodiode.* The sphere was calibrated by measuring the outputs of diodes of known efficiencies. The efficiencies of these ‘standard’ diodes were measured on several other calibrated integrating spheres within the laboratory. To avoid joule heating, the efficiencies were measured d.c. up to only 2 mA (1.5 A/cm*), and then current pulses were employed (20 psec width at a frequency of 1 kHz). Unless stated otherwise, the efficiencies quoted in this paper are external quantum efficiencies measured with a 100 mA pulsed current at 26°C for diodes embedded in epoxy domes. Table 2 summarizes the results of efficiency measurements and lists the various material parameters and fabrication processes for each group of diodes. The substrate (listed in the second column of Table 2) is the single most important factor in obtaining highly efficient diodes [ 11, and an evaluation of the quality of the GaAs ingot prior to growth is very useful. Two ingots (32 1 and 387) were evaluated using the photoluminescence test described .by Kressel et al. [l 11. Both ingots had high photoluminescent efficiency ratios *EGG,

Boston, Massachusetts.

Table 2 Average external efficiency (%) Al in melt Batch No. Substrate

d (pm) 8 6 6 8.5 6

LR-470 LR-5 14 LR-515 LR-517 LR-562

270 270 270 270 357

1.4 2.6 2.9 2 2.9

LR-573 LR-574 LR-575 LR-576

263B 387 18L 387

0.7 1.7 2.3 1.4

6 6 18 18

18L 387 357 18L G5-89 321 387

2.0 1.2 2.5 2.9 1.4 1.5 8.3

6 6 6 6 6 6 _

LR-577 LR-578 LR-584 LR-585 LR-586 LR-587 ZAG 380*

{

*Homostructure

3:1:1 No etch etch contact No dome No dome DOT DOT DOT DOT DOT H H

H thin n-GaAs H

diode, see Section 4(b).

H DOT DOT DOT DOT DOT

1.6 0.8 0.5 0.6 2.0 2.0 1.5 1.7 0.55 2.2 1.5 1.3 1.4 1.1 1.9 2.4 0.26

2.2 1.6 3.2 3.4

3:1:1 etch

EPOXY dome 3.0 5.1 2.7 5.3 8.0 9.0 7.0 7.4 2.0 7.5 7.0 6.0 5.0 8.3 6.6 9.0 8.0 1.0

Highest efficiency (o/o)

Pulsed diode current OnA)

4.2 5.7 3.2 5.5 10.0 10.0 7.5 7.8 2.2 7.9 7.6 7.7 5.8 9.8 7.0 10.1 9.5 1.3

200 20 20 20 100 100 200 200 200 200 loo 100 100 loo loo loo loo loo

GaAs-GaAlAs

ELECTROLUMINESCENT

(see Table 1) indicating high quality material and both produced reasonably efficient heterostructure diodes. Kressel’s observations regarding the relationship between laser efficiency and wafer location in the ingot [ 1 I] were also confirmed. For all diodes both the Ga,_,Al,As layer and the n-GaAs had nominal thicknesses of 50pm. In different wafers this thickness varied between 30 pm and 70 pm. In some cases the n-GaAs was intentionally polished to less than 20 pm for the purpose of heat-sinking the diode (LR-517 and some of LR-576). No relationship was evident between the thicknesses of these layers and the external efficiency. Diodes from LR-576 exhibited about the same efficiency regardless of whether they possessed - 45 pm or < 20 pm of n-GaAs. The effect of the 3 : 1: 1 HzS04 : H,Oz : H,O etch on the efficiency of the diodes is seen by comparing the efficiencies of diodes from wafers LR-514, LR-515, LR-517, and LR-562 (see Table 2). On the average, the 3: 1: 1 etch increases the efficiency of the diodes by a factor of 2. The clear epoxy dome with a refractive index of 1.53 usually increased the efficiency of the diodes by factors of 2 to 2.5. These values are close to the factor of 2.7 improvement expected assuming no absorption in the epoxy at 9100 hi. No effort was made in this work to use special films [ 141 or glasses with high refractive indices[l5] to reduce the internal reflection at the diode surface. The total increase in the external efficiency with the etch and epoxy dome is normally 300-500 per cent. Figure 4 shows the increase in light intensity obtained on one diode after the etching and the application of the epoxy dome. The effect of the etch on the current-voltage characteristic of the diode will be discussed later. As shown in Fig. 5 (open dots) the external quantum efficiency increases with increasing total current density up to a certain value and then remains nearly constant. The initial part of this increase can be explained in terms of decreasing nonradiative current components compared to the (radiative) diffusion current. This effect is most clearly illustrated by computing the efficiency on the basis of the diffusion current rather than the total current, and then plotting this efficiency as a function of the diffusion current - Fig. 5, solid dots. The diffusion current was obtained graphically by extrapolating the (radiative) n = 1 slope of the I-V curve back to low bias voltages (see Fig. 4).

1269

DIODES 1

1

I

I

I

LR-514-6

0

BEFORE

.

AFTER

ETCH ETCH

CURRENT ----DIFFUSION

CURRENT

---LIGHT

INTENSITY

-I

L 0.6

0.7

0.8 DIODE

0.9 VOLTAGE

1.0 (VOLTS)

4.f

1.2

Fig. 4. Diode current and light intensity vs. bias voltage at 300°K.

3

c $

10

!iI: ;g _I* @

g

as=

‘.O

-c 0

DIFFUSION CURRENT TOTAL CURRENT

w 0.1 10-z

10-I

100 CURRENT

40’ DENSITY

102

10’

(A/me)

Fig. 5. External quantum efficiency as a function of current density at 300°K. The open dots indicate the variation of the efficiency computed on the basis of the total current. The solid dots are efficiency values computed using only the diffusion current as explained in the text. For the diode shown in Fig. 5 the ‘diffusioncurrent external efficiency’ remains nearly constant up to - 2 A/cm2 and then increases by a factor of about 6 or 7 before saturating at - 70 A/cmz. Similar behavior is observed in all the heterostructure diodes, with the increase in the ‘diffisioncurrent efficiency’ between low and high current

1270

E. A. ULMER, JR.

levels ranging between factors of 3 and 7 in different diodes. It is unlikely that the saturation of the efficiency is produced by heating within the device because the efficiencies at 100 mA and 1000 mA (a.c.) are nearly the same while the input powers differ by an order of magnitude. The reason for this saturation of the external efficiency is not well understood at the present time. The temperature dependence of the external efficiency was measured in two ways: (1) an integrating sphere fashioned from a Dewar and a photomultiplier measured the relative efficiency at both 300°K and 77”K, and (2) the efficiency was measured with solar cells at fixed distances from the diode, which was mounted on a cold finger in an inverted-Dewar arrangement [ 163. In this second method, the emission intensity as a function of temperature was measured from 300°K down to - lOS”K, and 77°K values extrapolated from this data. The external efficiencies at 77°K increased by factors of 4 to 10 over their room temperature values. The amount of increase in efficiency was larger at the lower current densities. The highest external quantum efficiency recorded on a diode embedded in epoxy was 36 per cent at 77°K. This efficiency represented a factor of four increase over the room temperature value. Others have reported increases of 2[9], 3.5 [7], and 4[8] between room temperature and 77°K on homostructure GaAs diodes. At 400°K the efficiencies of the diodes decrease to about one-half of their room temperature values. Forward-biased diodes were examined visually with a low-power microscope equipped with an i.r. converter tube. Under forward-bias, the entire (15~mil)* diode appears uniformly bright, regardless of the type of p-product, dot or H, indicating that the emission is not confined to the region immediately beneath the p-contact. Results on LR-562 indicate that diodes with the H-contact may be slightly more efficient than those with the dot contact (see Table 2). The spatial distribution of the emission, a distance of 10 cm from an epoxy-domed diode, was determined using a solar-cell detector behind a narrow slit.. The roughly spherical emission (into a solid angle of 2) is illustrated in Fig. 6. The distribution indicates that emission from the side of the device contributes significantly to the total emission.

ANQLE

0

2 LIGHT

4 INTENSITY

FROM

NORMAL

6 6 (ARBITRARY

40 UNITS.1

Fig. 6. Spatial distribution of the spontaneous emission from a diode embedded in a clear epoxy dome. (b) Homostructure diodes In order to more clearly illustrate the improvement in efficiency realized with the heterostructure, we diffused Zn into a GaAs(Si) substrate normally used for epitaxy. The efficiency of electroluminescent diodes from this diffused wafer are listed at the bottom of Table 2. The efficiency is typical of those obtained with GaAs(Zn) diffused diodes, but is nearly an order of magnitude less than those obtained with the heterostructure. (c) Emission spectrum Normalized emission spectra are shown in Fig. 7. For current pulses of 700 A/cm2 at 300°K the emission is centered at 9080 A with a full-width at half-maximum (FWHM) of - 275 A (curve B). Joule heating in the diode under continuous operation at 700 A/cm* at 300°K shifts the spectral peak to 9140 A (curve C). The shift shown in Fig. 7 indicates a temperature rise of about 20°K at the junction under dc. operation. Lowering the temperature shifts the peak to higher energyat 88°K the emission peaks at 8540 A with an FWHM of - 250 A. (d) Frequency response The rise time and decay time of the spontaneous emission were measured with a photomultiplier possessing a time-constant of - 5 nsec. Current

GaAs-GaAlAs

ELECTROLUMINESCENT

pulses, 100 nsec long with rise times < 1 nsec and a frequency of 50 Hz, were applied to the diode. The measured rise times and decay times of the emission are about 25 nsec at 300°K and are nearly independent of the current density over the range 35 A/cm* to 1400 A/cm*. The response, however, was not strictly exponential or characteristic of a power law of the form P. Therefore the rise ENERGY 1.40

6400

8600

6600

(ev

1 1.35

9000

WAVELENGTH

9200

9400

9600

(ii,

Fig. 7. Electroluminescence emission spectrum for (A) 1 A ax., 88°K; (B) 1 A a.c., 300°K; (C) 1 A d.c., 300°K; where the temperatures are heat-sink temperatures. time (decay time) is defined to be the time necessary

to rise to (decay to) l/e of the peak intensity. Below 35 A/cm* the rise times of the heterostructure diodes increase and the decay times decrease. For example, at 15 A/cm* rtie = 80 nsec and T&w = 10 nsec. In contrast, the rise times and fall times of the homostructure diodes were 10 nsec independent of current density. These times, however, could not be measured easily, especially at the lower current densities, because of the low efficiency of the homostructure devices. (e) Electrical properties The current-voltage relationship of the heterostructure diodes is similar to that of other GaAs p-n junctions. As shown in Fig. 4, at low applied voltage the current is proportional to exp (qV/nkT) with n = 2 indicating space charge recombination. At moderate voltages, the thermal diffusion of electrons into the p-region produces the expected voltage dependence of exp (qV/kT) with n = 1. Figure 4 also shows that the 3 : 1: 1 etch significantly reduces the (nonradiative) surface leakage currents. Also, by eliminating the surface leakage currents with the 3: 1: 1 etch the reverse Z-Y

DIODES

1271

characteristic of all the diodes improves. The avalanche breakdown voltage in etched diodes is - 13 V, with reverse currents below breakdown of only a few tenths of a PA. The capacitance of the 15-mil* heterostructure diodes is on the order of 150 pF at zero bias, or lo5 pF/cm*. The dependence of the capacitance on applied bias is a(1/v1/3, which indicates[l7] the diffusion of Zn into the GaAs produces a graded junction. The impurity gradient is 0.4-1.5 X IF3 cmm4. (f) Aging studies Aging studies were performed in an effort to estimate the reliability of these heterostructure diodes. Diodes were bonded to tinned copper heat sinks and run continuously at acurrent density of 700 A/cm*. The presence of the tin precluded our using a prolonged etch to remove surface contamination. Some of the diodes had less than 10 pm of n-type GaAs between the junction and the heat sink, while others had 25 to 40 pm of n-type material. The light intensity was monitored with a solar cell permanently fixed in front of the diode. Normally the diodes were aged 500 hr at about 65°C in air. The amount of degradation for diodes from any single wafer varied widely. For example, in one group after 500 hr at 65°C several diodes had degraded to 50 per cent of their original output while another diode was still emitting at 85 per cent of its initial intensity. At the end of a life test, in addition to the light output, the I-V characteristic of the diodes is also degraded. That is, the nonradiative current component under forward-bias increased and the magnitude of the reverse breakdown voltage decreased. The results of the degradation studies are too widely scattered to draw quantitative conclusions. Surface contamination could be responsible for the degradation since the prolonged presence of the tin on the Cu blocks precluded any etch prior to the aging test. 5. DISCUSSION Nearly all of the radiation is generated in the compensated p-GaAs region since the diodes are operated well below the regime where hole-

injection into the n-region contributes significantly to the current [ 11. In the p-GaAs the compensation and the high impurity concentration both act to

1272

E. A. ULMER, JR.

reduce the effective bandgap* from its normal 300°K value of 1.43 eV. The emission at 1.36 eV is produced by transitions from the conduction band to tail states in the valence band. Optical absorption measurements [20,2 l] indicate the absorption coefficient at 9 100 A is approximately 12 cm-’ for n-type GaAs doped to 3 X 1018 cmes. This low value means the emission from the compensated p-layer is not strongly absorbed in the n-GaAs portion of the device. The alloyed Sn-Ni-Au n-contact on the diodes is probably not a good reflector, and a more highly reflecting n-contact might increase the external efficiency. In the p-GaAs the absorption coefficient reaches lo4 cm-l for energies greater than I .36 eV. In fact, such selective high-energy absorption could be responsible for the slight asymmetry observed in the emission spectrum (see Fig. 7). However, the overall effect of absorption in this thin layer is difficult to estimate because of the rapid variation of the absorption coefficient in the vicinity of the energy gap. Two features unique to these heterostructure diodes also help explain their high external quantum efficiency: (1) the Ga,_,Al,As layer is nearly transparent to the GaAs emission, and (2) the GaAs-Ga,_,Al,As interface is nearly free of nonradiative recombination centers. Electrons injected into the p-region recombine radiatively within 2 pm of the transparent Ga,_,Al,As. The fact that the heterostructure diodes are nearly an order of magnitude more efficient than the homo-

*In the heavily compensated p-region both the Coulomb and exchange interactions shift the band edges. The Coulomb interaction lowers both band edges although not necessarily by equal amounts. Since this difference between the Coulomb shifts is not known, the shifts will be presumed equal[l8]. The exchange effect, on the other hand, shifts the band edges towards each other, thereby reducing the bandgap. The conduction band is lowered about 35 meV by the exchange interaction. The upward shift of the valence band cannot be calculated explicitly[ 181 but is probably in the neighborhood of 20-30 meV. These energies were obtained using equations given by Hwang[ IS] assuming NA = 6X 10ls cmW3and ND = 3 X lOI*cm-3, and using a value of 4 x lo6 cm-l for the reciprocal screening length at the relatively low injection levels under consideration (< IO3A/cm*)[ 191. As a result of these effects the bandgap in the p-region of the heterostructure diodes is reduced to about 1.38 eV. At these high doping levels the bandgap is effectively reduced still further because the Zn-impurity band is merged with the valence band.

structure diodes shows that the structure contributes to the high efficiency of these diodes. The fourfold increase in the external efficiency at 77°K over the room temperature value is similar to increases previously reported for GaAs homostructure diodes [7-91. Both Ashley and Strack[7] and Gonda et al. [22] have presented evidence for GaAs(Si) homostructure diodes which shows that a significant reduction in the internal absorption is obtained as the diode is cooled from 300°K. It is possible that the increase in the external efficiency of the heterostructure diodes is determined by the reduction in internal absorption in the GaAs portion of the device. GaAs optical absorption data indicates a reduction in absorption at the emission wavelength by a factor of - 5 in both the n-[21] and pregions[20] of GaAs doped near 3 X lo’* cm+ when the temperature is reduced from 300°K to 77°K. It is also possible that the internal quantum efficiency may increase slightly at lower temperature and thus contribute to an increased external efficiency. For example, Hill [23] has reported room-temperature values for the internal quantum efficiency of 34 per cent and 63 per cent, and both Hill[23] and Keyes et al.[24] have estimated the internal efficiency at 77°K to be near unity. The measured response times of 25 nsec for these heterostructure diodes are similar to the 10 nsec reported by Alferov et al. [lo] for heterostructure diodes. Both of these values are much larger than the 1.6 nsec rise time reported by Carr[25] for GaAs diodes formed by diffusion of Zn into Sn-doped substrates. We note that long response times have recently been reported[26-281 for Si-doped GaAs homostructure diodes and are attributed to the close compensation. However, the heterostructure diodes are not closely compensated[l]; and, hence, this mechanism cannot be invoked to account for our long response times. 6. SUMMARY AND CONCLUSIONS GaAs-Ga,_,Al,As electroluminescent

single

heterosttucture

have been fabricated and their properties described. External quantum efficiencies as high as 10 per cent and continuous power outputs up to 65 mW at 9100 A have been obtained on these solution grown planar diodes. The order of magnitude increase in the efficiency over normal Zn-diffused (homostructure) diodes is attributed to several features of the heterodiodes

GaAs-GaAlAs

ELECTROLUMINESCENT

structure design. The efficiency increases with increasing current density up to - 70 A/cm2 and then remains constant. A calculation[29] of the expected external efficiency indicates the roomtemperature internal quantum efficiency of these heterostructure diodes is over 50 per cent. At 77°K the efficiencies increased by a factor of about four at higher current densities. The highest external efficiency measured at 77°K was 36 per cent. The electrical characteristics of the diodes are, not unexpectedly, similar to other GaAs diodes since the junction is located in the GaAs. The time response of the emission, however, showed the diodes were unlike other Zn-diffused GaAs devices in that they possessed rather long rise and decay times of - 25 nsec. Acknowledgments-The skilled help of Mrs. D. R. Ketchow in many phases of this work, especially the solution growth, is gratefully acknowledged. The author is indebted to L. A. D’Asaro, J. C. Dyment, J. E. Ripper, T. L. Paoli, C. J. Hwang, J. H. Rowen, and J. S. Jayson for numerous discussions and suggestions; N. E. Schumaker and R. A. Fumanage for assistance in the fabrication, I. Hayashi for photoluminescence measurements, and W. H. Hackett, Jr., for assistance in designing and calibrating the integrating sphere. REFERENCES 1. I. Hayashi and M. B. Panish,J. uppl. Phys. 41, 150 (1970), and references therein. 2. H. Kressel. H. Nelson. and F. Z. Hawrv1o.J. anal. _, .. Phys.41,2dl9(1970). 3. A. H. Herzog, Solid-St. Electron. 9, 721 (1966), and references therein. 4. T. Nakano, K. Fujikawa, and T. Oku, Jap. J. appl. Phys. 6,665 (1967). 5. R. J. Archer and D. Kerps, Proc. 1966 GaAs Symposium, Reading, p. 103, Institute of Physics and Physical Society, London (1967).

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