MID-INFRARED OPTOELECTRONICS: MATERIALS AND DEVICES

infrared li E

SSKizhayev, N.V.Zotova, S.S.Molchanov and Y.P.Yakovlev

Abstract: Light emitting diodes (LEDs) are fabricated on the basis of MOVPE-grown N-InAsSbPln-InAsSblP-InAsSbP heterostmctures. LEDs operating in the 3.454.45 pm wave- length range at room temperature are produced by employing InAsSb active layers with different Sb content. The photoluminescence of InAsSb layers and electroluminescence properties of LEDs are investigated. LED light-current characteristics in both quasi-continuous wave mode and pulsed mode are also studied. Increasing LED efficiency with improved carrier confinement is observed. When operating at 50% duty cycle, the LEDs (room temperature wavelength 1, = 4.27 pm) produce an average power of 0.3 mW when driving an average current of 0.25 A. With a peak current of 1.4 A (5% duty cycle), the pulse power of the diodes (room temperature wavelength 3, = 4.27 pm) is measured as 2.5 mW.

1 Introduction

Infrared emitters operating in the 3-5 pm spectral range are very important for a wide range of applications, such as chemical sensing for atmospheric pollution, chemical process control, and some military applications. Semi- conductor light emitting diodes (LEDs) have a great advantage over filament sources due to their lower electric power consumption and higher spectral radiation density. In addition, the operation of filament bulbs is limited to t 4 pm because of the glass envelope of the source. Being more convenient and cost effective than laser diode opera- tion under cryogenic cooling conditions, LEDs are much more feasible for portable gas analysers.

Until recently, liquid phase epitaxy (LPE) was the most attractive growth technique for the production of light emitting diodes for this spectral range [I-61. Only a few reports dealt with LED manufacturing by metalorganic vapour phase epitaxy (MOVPE) [7, 81, and molecular beam epitaxy (MBE) [9-111. The LPE growing of an InAsSbP solid solution is restricted by a large immiscibility region, resulting in an insufficient carrier confinement in an InAsSbP/InAsSb/InAsSbP double hetero-structure (DH). In contrast, MOVPE is devoid of this deficiency, and allows growth through the whole range of phosphorous content (up to 70% in an InPSb solid solution). Although AlSb-containing layers are the best electron barriers in antimonide-based devices, the MOVPE growth of InAsSbP alloys is much simpler due to MOVPE specific problems with A1 precursors, and because suffi- cient carrier confinement in an InAsSbP/InAsSb/InAsSbP heterostructure can be provided.

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0 IEE, 2002 IEE Proceedings online no. 2002017 I DOI: IO. 1049/ip-opt:20020 17 I Paper first received 31st July 2001 and in revised form 8th January 2002 The authors are with the Ioffe Pkysico-Technical Institute, Polytechnicheskaya 26, 194021, St. Petersburg, Russia

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In the present work 'we deal with MOVPE-grown LEDs covering the 3.45-4.45 pm wavelength range at room temperature with average output powers of 0.42 mW (3, = 3.45 pm), 0.3 mW (2 =4.27 pm), and 0.125 mW (A = 4.45 pm) in quasi-continuous wave mode (50% duty cycle). The results of electroluminescence measurements in pulsed mode, as well as of photoluminescence measure- ments of InAsSb layers, are also presented.

2 Experiment

The growth is carried out in a typical horizontal flow MOVPE reactor operating under atmospheric pressure conditions. Trimethylindium (TMIn), trimethylantimony (TMSb), arsine (AsH3), phosphine (PH3), diethylzinc (DeZn) are used as precursors. Hydrogen is used as a carrier gas at a flow rate of 18 l/min. (100) and (1 1.1)- oriented InAs substrates Te-doped up to the level of n - 3 x 10" cmP3 are used for growth. The LED structure consists of an undoped InAsl _xSbx active layer surrounded by undoped ~ - I ~ A S ~ . ~ ~ Sb0.12P0.~~ and doped p+- InAso,61Sbo,12Po,27 (Zn-doped up to. p - 2 x 10" ~ m - ~ ) cladding layers 0.5 pm and 2 pm in thickness, respectively. The active layer is 2.5 pm thick in all structures. DH con- taining XSb - 0, 0.-105, 0.157 in InAs,-,Sb, are grown. In addition, the barriers composition is varied for DH. with an active layer containing XSb - 0.105, in order to evaluate the increasing LED efficiency with improving carrier confinement. DH with I n A s 0 . 2 9 S b ~ . ~ ~ P ~ . ~ ~ barriers are grown. An energy band diagram of the LED heterostruc- ture is shown in Fig. 1. According to our estimation based on the model proposed in [12], the bandgap energy Eg of quatemary confinement layers InAs0.29Sb0.2jP0.4~ is 0.57eV. Eg of the active layer varies between 0.36 and 0.28eV upon changing the antimony content XSb. The thickness of the layers grown is determined by cross sectional transmission electron microscopy. Confinement layers are lattice matched to the InAs substrate with a lattice mismatch Aula of less than 6.5 x lop4 confirmed by X-ray analysis. The component composition of the solid

IEE Proc.-Optoelecti-on., Vol. 149, No. I , Febvtiay 2002

Fig. 1 Schematic energy bund diagram of LED heterostructure

solutions is determined using a CAMECA CAMEBAX X-ray microanalyser.

The LEDs are fabricated as mesa-diodes of 300pm in diameter by the conventional photolithography technique. A continuous nonrectifying contact is formed on the top of the epitaxial structure. The point contacts on the substrate side are 100 pm in diameter. The contacts are formed by the vacuum evaporation of gold and tellurium onto an n-type layer or of gold and zinc onto a p-type layer. The total thickness of the LED chip is 200 pm, and the p-type area of the LED structure is indium soldered onto the standard case. This construction provides sufficient heat dissipation. In addition, a parabolic reflector is mounted on the case in order to narrow the angular distribution of the radiation down to 10-12". The properties of the structures grown are studied using photoluminescence (PL) and electroluminescence (EL) methods. The PL properties are studied at T= 77 K. The emission is recorded using a cooled TnSb photodiode and a lock-in amplifier. The monochromator IKM-1 is used as dispersive element. The PL is excited by a GaAs diode laser (wavelength 2 = 0.8 pm, output power in pulse mode 10 W, z = 5 ps, and f = 500 Hz). The laser beam is directed to the grown solution in the reflection mode.

The LED EL is studied both in the pulse and quasi- continuous wave modes at room temperature. The radiant power is measured using a standard Nova instrument equipped with a 2A-SH thermocouple sensor. At constant repetition frequencyf= 500 Hz, the pulse duration z varied through 1000, 500, 200, 100 and 501s. The pulse output power is obtained as the average power measured by the Nova instrument multiplied by the off-duty factor.

3 Results and discussion

Let us consider the growth procedure of the emitting structures along with the PL characterisation of the grown layers. Vapour pressures of 2.97mmHg for TMI (25"C), of 22.43 mmHg for TMSb (-6"C), of 5.0mmHg for DeZn (5°C) are used for calculation of the V/III and TMSb/(TMSb + ASH,) ratios, and the DeZn partial pres- sure. In order to reduce the residual carrier concentration in the active layer and to increase the LED quantum effi- ciency, several experiments on InAs deposition are carried out. Since InAs appears to grow best at V/III ratio of 40, and nucleation temperature 610°C at a growth rate of 0.4 pmlmin, InAs films have a featureless surface morphology. Fig. 2 shows PL of InAs epilayers 1 pm in thickness. Superlinear growth of the InAs layers' PL line intensity with increasing excitation power is observed (inset to Fig. 2). PL spectral curves exhibit narrowing of the full width at half maximum (FWHM) from 13 meV to 10 meV as a result of four-fold increasing excitation

IEE Pr~~c.-Optoelectron., Vul. 149, No. I , February 2002

energy, meV

Fig. 2 Photoluminescence spectra (T= 77 K) of three samples: InAs, InAso,svsSb~ 105 and lnAso.84~SboIs7 (inset: photoluminescence line inten- sity against e.ucitation intensiiy of InAs epilayers)

intensity. Narrowing of the PL spectral curves and superlinear growth of PL line intensity with increasing excitation power can serve as an indication of super- luminescence.

Our aim is to achieve LED high-power emission on wavelengths of 3.45, 4.27 and 4.45 ym at room tempera- ture. The model proposed in [12] is used to calculate the active layer composition required for acquisition of the above-mentioned wavelengths. For our LEDs to operate at room temperature on 3.45, 4.27 and 4.45 pm, respectively, the antimony content should be 0, 10.5 and 15.7%. For InAsSb growth the temperature is lowered to 575"C, due to the low melting points of these compositions [ 131. Fig. 3 illustrates the antimony content dependence in the solid phase on the TMSb/(TMSb+AsH3) ratio in the vapour phase. It should be noted that the V/III ratio must be sufficiently low in order to avoid the formation of defects on the growing surface. In the presence of excess Sb in the vapour phase, elemental Sb would be deposited on the substrate surface. Fig. 2 shows PL spectra of three samples of InAs, InAso.895Sbo.lo5 and InAso.843Sbo. 157, which are 1 pm in thickness. Peak positions for InAs (408meV), InAso.895Sbo. 105 (335 mev), InAS0.843Sbo. 157 (326 meV)

0 0.2 0.4 0.6 TMSb'N

Fig. 3 Dependence of antimony content in solid phase on TMSb/(TMSb + AsH3) ratio in vapour phase

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correspond to the bandgap of the ternary solution at 77 K. Calculations using the lnAs bandgap thermal coefficient dE,/dT= 2.8 x eV/K give the desirable wavelength at a room temperature wavelength for each ternary solution mentioned above. With increasing Sb content in the ternary solution the PL intensity drops considerably due to the increasing lattice mismatch between the InAs substrate and InAsSb epitaxial layer, and due to the rising role of the Auger recombination mechanism. 77 K PL spectra FWHM = 36, 42 meV (-4kT) for InAso.895Sbo.Ios and I ~ A s ~ , ~ ~ ~ S ~ ~ , I 57 are correspondingly also typical for InAsSb. Sharp PL spectra of InAs (FWHM = 10 meV), comparatively low values of FWHM for InAsSb, the absence of defect induced transition lines, and a decrease of the short wavelength shoulder of the PL spectra as

in the whole interval of Sb concentration are all evidence both of the low level of background doping density, and of uniform crystallinity, as well as of a high- quality interface between adjacent epitaxial layers.

InAsSbP cladding layers are grown in the temperature range 520-575°C. A hydrogen flow of 580cm3/min through the TMIn bubbler kept at 25°C and arsine flow of 3 cm3/min are used for growth. Vapour phase ratios are

Details of the InAsSbP growth will be reported elsewhere. Fig. 4 illustrates the results of electroluminescence

measurements at room temperature for three LEDs with the following active layer compositions: InAs, I n A s ~ . ~ ~ ~ S b o . 105 and I n A s 0 . 8 ~ ~ S b ~ . 157. The cladding layer composition was InAso.61 Sb0.12P0.27. The LED peak loca- tions correspond to the active layer bandgap and are independent of the injection current at 50% duty cycle conditions in the current range 0-0.2A. LED radiation absorption by C02 (A =4.27 pm) is quite clear from the emission spectrum. As in the case of PL measurements, EL intensity drops and FWHM is widened with increasing Sb content in InAsSb because of the rising lattice mismatch between the substrate and the adjacent epitaxial structure.

Figs. 5-7 show LED light-current characteristics for several wavelengths in a pulse regime. The light-current curve in a quasi-continuous regime (Fig. 8) shows a 2.5-fold improvement in LED efficiency (A = 4.27 pm) with a wider bandgap barrier ( I ~ A s ~ . ~ ~ S ~ ~ . ~ ~ P ~ . ~ ~ incorpo- rated instead of InAso.61Sbo.12P0.27). It is beyond reason to expect that the uptrend of LED efficiency with wider bandgap cladding layers would cease for the diodes oper- ated at other wavelengths. For 50 ps pulse duration, a pulse output power exceeding 5 mW driven by a current of 1.4 A is achieved for LEDs operating at wavelength A = 3.45 pm.

TMSb/(TMSb +AsH3 + PH3) 10.15-0.45; V/IIl= 2-5.

. . . . . InAso 61Sb0 12Po 27/lnAs/lnAso 12Po 27 InAs061Sb0 12P027/~nAs0895Sb0 105/1nAs061Sb0 12‘027

- - - lnAs06lSb0 12P027/lnAS0843Sb0 157/1nAS061Sb0 12‘027

-

I

.: .$ a, c -

0 3 4 5 6

A. prr Fig. 4 Electroliiniinescence measurements at room temperature for three LEDs with active layer compositions InAs, lnAso ,~y~Sbo. lo~ and In& 843Sb0 157

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0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 peak current, A

Fig. 5 Room temperature light-current characteristics of LED with i = 3.45 pm (repetition kequency f = 500 Hz, pulse duration r = 1000, 500, 200, 100 and 50 psj

&r=500 ps

+r=lOO ps

L” a,

1.5

- % 2 1.0

0.5

n 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

peak current, A

Fig. 6 Room temperature light-current characteristics of LED with 2. = 4.2 7 p m (repetition frequency f = 500 Hz, pulse duration T = 1000, 500, 200 and 100 ps)

0.8 - 0.7 -

2 0.6 -

6 0.5 - 3 0.4 - 1 0.3 -

0.2 -

0.1

Q

-

&r=IOOO ps

0 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

_J

1.6 peak current, A

Fig. 7 Room temperuture light-current characteristics of LED with i. = 4.45 pm (repetition frequency f = 500 Hz, pulse duration r = 1000, 500, 200 and 100 ,is)

m = ~nAs0.29Sb0.23P0.48

i 5 0.20 n

0.05

0.4 0.6 0.8 1 .o 1.2 0 0.2

peak current, A

Fig. 8 Room temperature light-current characteristies in quasicontinu- ous regime of two LEDs ( i=4.27pm) with d$erent cladding laver composition (50% duty cycle regime)

IEE Proc.-Optoelectron., Vol. 149, No. I , February 2002

Values of 2.5 mW (InAso 29SbO 23Po 48 barriers are used), and 0.75mW are obtained for wavelengths of 4.27 and 4.45 pm, respectively, at the same current for 100 ps pulse width. Non-radiative Auger recombination CAuger n3 is probably responsible for the output power saturation at high current. Joule heating also affects the LED operation. As is obvious from light-current characteristics, the output power saturation occurs at higher current, with the pulse width being reduced.

Despite the high values obtained in a pulse regime, the output power in a quasi-continuous regime needs further improvement. One way toward this improvement is an increasing carrier confinement through growing InAsSbP cladding layers with a higher phosphorous content or incorporation of lnGaAsSb or AlGaAsSb barriers for better electron confinement. Nevertheless, these diodes are very promising for gas sensor applications.

4 Conclusions

As a result, N-InAsSbPIInAsSblP-InAsSbP LEDs have been produced by MOVPE. We have examined 77K PL of lnAsl-,Sb, layers with antimony content XSb - 0, 0.105 and 0.157. Electroluminescence measurements at room temperature, together with output power estimation, were held for LEDs. An improvement of LED efficiency (2.5 times) was observed by using I ~ A s ~ , ~ ~ S ~ ~ . ~ ~ P ~ . ~ ~ barriers instead of InAso,61 Sbo,12Po,27, layers. The non-radiative Auger recombination mechanism and the Joule heating effect are responsible for the power saturation at high current. Average optical power values of 0.42mW (A = 3.45 pm), 0.3 mW (2 = 4.27 pm) and 0.125 mW (A=4.45 pm) were obtained for LEDs in the quasi- continuous regime. Pulse output powers of 5 mW (I , = 3.45 pm), 2.5 mW (3. = 4.27 pm) and 1 mW (A = 4.45 pm) were achieved. These devices show promise for use in portable gas analysers.

5 Acknowledgments

This work was supported in part by the Russian Basic Research Foundation, grant 00-02-17047. S.S. Kizhayev

wishes to thank the Robert Havemann Foundation for financial support.

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