Magnesium doping in In0.32Ga0.68P grown by liquidphase epitaxyChyuanWei Chen, MengChyi Wu, and LiKuang Kuo Citation: J. Appl. Phys. 71, 4475 (1992); doi: 10.1063/1.350791 View online: http://dx.doi.org/10.1063/1.350791 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v71/i9 Published by the American Institute of Physics. Related ArticlesHigh-Q optomechanical GaAs nanomembranes Appl. Phys. Lett. 99, 243102 (2011) Mg-induced terahertz transparency of indium nitride films Appl. Phys. Lett. 99, 232117 (2011) Effect of metal-precursor gas ratios on AlInN/GaN structures for high efficiency ultraviolet photodiodes J. Appl. Phys. 110, 103523 (2011) Pulsed terahertz emission from GaN/InN heterostructure J. Appl. Phys. 110, 103103 (2011) Suppression of nonradiative recombination process in directly Si-doped InAs/GaAs quantum dots J. Appl. Phys. 110, 103511 (2011) Additional information on J. Appl. Phys.Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors

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Magnesium doping in In 0.32Ga0.68P grown by liquid-phase epitaxy Chyuan-Wei Chen, Meng-Chyi Wu, and Li-Kuang Kuo Research Institute of Electrical Engineering, National Tsing Hua University, Hsinchu, Taiwan 30043, Republic of China

(Received 4 December 1991; accepted for publication 27 January 1992)

Mg-doped Ine3,Gac,sP epitaxial layers have been grown on (100) GaA~c.~~Pc,,s substrates by liquid-phase epitaxy using a supercooling method. The electrical properties of the Mg-doped layers are investigated using capacitance-voltage methods at 300 K. The full width at half maximum value of the 300-K photoluminescence spectrum increases with hole concentration for Mg-doped layers. The 77-K photoluminescence spectra show, two peaks and one broad band, and their relative intensities change with various hole concentrations. The Mg acceptor ionization energy obtained from 77-K photoluminescence spectra is in the range from 20 to 38 meV. Finally, the photoluminescence spectra of Mg-doped In 0,32Gac6sP layers with three hole concentrations measured at various temperatures between 20 and 150 K are presented for the

- gradual evolution of the peaks.

I. INTRODUCTION

Visible semiconductor lasers in the 600~nm wavelength region have many attractive applications as light sources in optical information processing systems and optical fiber communication systems with plastic fibers. Shortening of the device wavelength is expected for obtaining small fo- cused spots and highly luminous light sources for human vision.

Recently considerable attention has been given to in- dium gallium phosphide (InI -.Ga,P) as an efficient lu- minescent and laser material throughout the red, orange, and yellowish green regions of the spectrum because of its large direct band gap extending up to 2.26 eV (x-0.74) at 300 K.’ It can be grown on commercial GaAse61P,,,, ep- itaxial substrates with an exact lattice match at a compo- sition of 68% GaP and a band gap of about 2.17 eV at 300 K.2*3 It is also a key material for AlInP/InGaP/AlInP, InGaP/AlGaInP/InGaP, InGaP/InGaAsP/InGaP, and InGaP/AlGaAsP/InGaP heterostructure visible laser sys- tems. In fabricating laser diodes and light-emitting diodes (LEDs) consisting of InGaP layers, the material must be successfully doped with donor and acceptor impurities. Al- though InGaP LEDs or laser diodes fabricated on GaAsP substrates by liquid-phase epitaxy (LPE)“8 or vapor- phase epitaxy ( VPE)9.‘0 have been reported, there are only a few reports on their doping properties. Kato et ~l.“~‘* reported the electrical and photoluminescence (PL) prop- erties in solution-grown polycrystalline Int _ XGaXP doped with Zn. Okuno et aLI have observed the electrolumines- cence spectra from In, _ XGaXP p-n junctions both in the direct- and indirect-band-gap regions at 77 K and at higher temperatures. Kim and Moon14 reported the properties of Zn diffusion in the In, -XGaXP alloy and pointed out the diffusion depth decreases with increasing the Ga solid com- position in InI _ XGaXP. Fujimoto et al.” reported the dop- ing properties of Te and Zn in Ino.26Gae74Aso.osPo,9~ (4 - 2.17 eV) grown on G~As~.~~P~.~~ substrates by LPE, and concluded that they have the segregation coefficients of 0.009 and 0.035, respectively. We previously reported the doping properties of Te and Zn in Inc3zGacbsP layers

grown on GaAso.61Po.39 substrates by LPE in detail.16 However, with the high vapor pressure and high diffusion coefficient of Zn it is very difficult to obtain an efficient control of the hole concentration and the precise p-n junc- tion location. Mg is expected to be more attractive than Zn as a p-type dopant in GaAs and related III-V compounds because the diffusion coefficient of Mg is about lo* times lower than that of Zn in GaAs.t7 But there are no reports on the electrical and optical properties of Mg-doped Ino.32Gac6sP material or devices.

In this paper, we report the doping properties of mag- nesium in Ino.s2Gac6sP layers grown at 800 “C by LPE. Their electrical and optical properties are examined. The temperature dependence of PL from Mg-doped Ino,32Gac6sP layers with various hole concentrations is de- scribed. Various luminescence bands involving donors and acceptors are identified, and ionization-energy values are deduced from the data.

II. EXPERIMENT

In1 _ XGaXP epitaxial layers were grown by LPE in a sliding boat under a flowing hydrogen ambient. The sub- strates were GaASo.61Po,39 Commercial epitaxial wafers grown by VPE on S-doped (100) GaAs substrates 2” off toward (110). A -30~pm-thick graded composition GaAsP layer is used between the GaAs and GaAscslPo.39 layer to avoid a large lattice mismatch. The surface GaAso,61Po.39 layer is 40 pm thick and n doped to 7X lOI cm - 3 with Te. All the In solvents used in the growth were first prebaked for 10 h or more at 900 “C. After the baking process, 20.2 mg of InP and 12.7 mg of Gap, both polycrystalline, and an appropriate amount of Mg were added to the 3-g In prebaked melt to form the growth solution with a liquidus temperature of - 8 10 “C. In order to accurately control the hole concentration, In- 1% Mg wires were used as a p-type dopant in this study. The doped epitaxial layers were grown by means of super- cooling technique with - 10 “C supersaturation, which is the best growth condition determined previously.3 The

4475 J. Appl. Phys. 71 (9), 1 May 1992 0021-8979/92/094475-06$04.00 @ 1992 American Institute of Physics 4475

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‘: E ” 0 I

z 5 F 17- IO - 2 Y 5 u

Mg -doped lno,32 Gao.,eP 300 K

I 016' I I IO8#O,I I 1 I11110 Id5 I o-4 la3

Mg MOLE FRACTION IN GROWTH SOLUTION, x&

FIG. 1. Room-temperature hole concentration of Mg-doped Inc32Ga,,68P layers grown on GaAs,,61Po.~9 substrates vs Mg mole frac- tion in the growth solution, X&

thickness of InGaP epitaxial layers grown during a fixed growth period of 5 m in was typically 4 pm. Capacitance- voltage (C-V) and PL measurements were carried out to characterize the Mg-doped IncszGae6sP layers. Details of growth conditions and characterization techniques were given elsewhere.3”6~18

Ill. RESULTS AND DISCUSSIONS

For all the Mg-doped 11r,,~~G~,~sP samples, the sur- face morphology is very shiny and flat but with a so-called cross-hatched-pattern surface inherited from the original m isfit dislocations introduced during the graded layer growth of GaAsP on the GaAs substrate.” The interface between the epitaxial layer and substrate is also flat and free from inclusions. Lattice m ismatch between the epitax- ial layers and GaAsP substrates normal to the wafer sur- face of the doped samples is controlled to be within + 0.15%-0.20%, which can result in the best layer

quality.3 The undoped (n-type) Incs2Gac&sP sample with a

background concentration of 1 X 1016 cm - 3 is used as a reference. Figure 1 shows the room-temperature hole con- centration of a Mg-doped IncszGa#-&sP layer grown on G~As~.~~P~,

7 9 substrate versus Mg mole fraction in growth

solution, X, . The hole concentration increases linearly from 5 X 10” to 4.2~ lo*’ cm- 3 as J&s varies from 2.6 X 10 - ’ to 2 X 10 - 4 mole fraction, The segregation co- efficient K of the dopant Mg in Inc32Gac6sP layers grown at 800 “C is estimated as 0.042 assuming that all dopants are ionized. However, this value of KM8 is three times smaller than that of 0.12 for Mg-doped IncsGacsP layers2’ and is very close to those previously reported values of 0.035 and 0.050 for the Zn-doped Inc26Gac.74Asc.osPo.95

T 90 I I I IIIIII I I I IIIII

E Mg -doped ‘“0.32 G”0.66 p 5 300 K

I ao x 2

5 60 E D 3 T: 50. I?

2 404 IOlb lOI IO'"

HOLE CONCENTRATION , p (~rn-~)

FIG. 2. Full width at half maximum of 300-K photoluminescence peak in Mg-doped IncJ2GacWP layers as a function of hole concentration.

(Ref. 15) and Inc~zGac6sP (Ref. 16) layers grown on GaAsc6tP0.3s substrates, respectively. The difference may be due to the various layer compositions and residual im- purity concentrations.

At room temperature, only one emission band due to the recombination of free electrons and free holes is ob- served in the PL spectra for Mg-doped Ino.32Gac,68P layers, as seen from the emission band due to free-electron-to-free- hole transition in the undoped Inc~2Gac6sP layers. The PL peak wavelength of the Mg-doped Ino.32Gac6sP layers is located at around 5760 A, which is slightly larger than that of 5730 A for the undoped layers. It increases slowly with increasing hole concentration due to the evolution of ac- ceptor impurity levels into a band and merging with the valence band. The full width at half maximum (FWHM) of room-temperature PL peaks in Mg-doped Incs2Gac6sP layers as a function of hole concentration is shown in Fig. 2. The FWHM increases from 52 to 83 meV in the hole- concentration range between 5 X lOI and 4.2 X 10” cmv3. In undoped IncszGac6sP layers, 36- and 11.5-meV FWHM values were obtained at 300 and 16 K, respec- tively.

The PL emission spectra at 77 K with various hole concentrations are presented in Fig. 3. These spectra are normalized to the same main peak intensity. Except for the undoped and most heavily doped samples, as shown in Figs. 3 (a) and 3(f), all other samples exhibit two peaks and one broad band denoted as A, B, and C. Peak A with the narrowest FWHM and the highest photon energy is attributed to the near band-to-band transition because of the sharpness of this peak and its peak position.“3 Except for the sample shown in Fig. 3 (f), the fact that the position of peak A holds constant for all the spectra indicates the composition of all the epitaxial layers is held constant and uniform. The lower-energy peak B is located at 20-38 meV below peak A and moves closer to peak A with increasing hole concentration. Since the relative intensity of peak B

4476 J. Appl. Phys., Vol. 71, No. 9, 1 May 1992 Chen, Wu, and Kuo 4476

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m 2

:: 5 2 k

t!i E z Y s 2 c 0

2

II A Mg -doped

I %.3zGaa.68

77-K PL

(a)

(b)

n (e)

/ I I I I IO 5500 5700 5900 6100 6300

WAVELENGTH ( % 1

FIG. 3. 77-K PL spectra of Mgdoped InoSzGao6sP layers grown on GaAso61Po,,P substrates with a carrier concentration of (a) n = 1 X 1016 (undo+), (b) p = 5 X 1016, (c) 1X10”, (d) 2x10”, (e) 3 x lo”, and (f) 4.2 x 10” cm - ‘. The spectra are normalized to the same main peak intensity.

increases with hole concentration, this peak is definitely associated with the transition from conduction band to Mg acceptor level. We can thus deduce that the high-energy peak at -2.229 eV ( -5560 A) as shown in Fig. 3(f) is due to the acceptor impurity band merging with the va- lence band. The transition of band C will be discussed later.

It is well known that the impurity ionization energy is a function of dopant concentration. The value of the ac- ceptor ionization energy EA can be estimated to the form2’

EA = Eg - hvB,

where Eg is the band-gap energy of In,-,s2Gac6sP and hvB is the peak energy of the Mg acceptor level. Figure 4 shows the quantities EA determined at 77 K as a function of P 1’3 for the Mg-doped I1re3~Gachs P layers. It shows a lin-

60

s E ‘; 50 W

.

&

6

iz 40

i5 F s 5 30 0

E E-J 20

3

IO

.

77K

. . .

. . ‘“o.3zGao.eeP

1 I I I I

4 5 6 7 8 9

p”3( x 105cm-‘)

FIG. 4. Acceptor ionization energy determined at 77 K in Mg- and Zn-doped Ino,,zGa,,6sP layers plotted as a function of the cube root of the room-temperature hole concentration.

ear decrease of the acceptor ionization energy as the cube root of the hole concentration is increased and can be rep- resented as the following empirical expression22:

EA = PA - ap113,

where I$ is the ionization energy for infinite dilution and a is a constant. Calculated values for I$ and a are 55.6 meV and 4.7 x 10 - ’ meV cm, respectively. The EA value of Mg varies from 38 to 20 meV as p varies from 5 X lOI to 4.2 X lOI cm - 3. Figure 4 also shows the 77-K EA values for Zn-doped In0,s2Gac.6sP layers as a function of p1’3 by the dashed line. The quantities @A and a corresponding to the Zn impurity are 60 meV and 1.5 x 10 - 5 meV cm, re- spectively. It can be observed that the ionization energy of the acceptor Mg is smaller than that of Zn by 15-27 meV in the In0,32Gac6sP layer. This smaller EA value for the Mg dopant than Zn was also obtained in the Al,Ga, _ xAs system.23*24

Figure 5(a) presents the temperature dependence ( T = 20-150 K) of emission spectra from the Mg-doped sam-

ple with a hole concentration of 5 x lOI cm - 3. No signif- icant changes take place when the temperature is lowered from 300 to 120 K, as in the case of the undoped InGaP layer. Peak B, the transition from the conduction band to the Mg acceptor level, starts to merge as a shoulder of peak A in the low-energy side at 100 K and its relative intensity gradually increases when the temperature is reduced. As the probability of liberation of nonequilibrium electrons from the conduction band to the acceptor level becomes much higher, the relative intensity of peak B to peak A is consequently increased. A decrease in the temperature be-

4477 J. Appl. Phys., Vol. 71, No. 9, 1 May 1992 Chen, Wu, and Kuo 4477

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low 50 K gives rise to a long-wavelength band C, which is located in the wing of peak B. The C-band intensity in- creases when the temperature is further decreased. At the same time, band C shifts rapidly toward a shorter wave- length.

The PL spectra of the Mg-doped samples with a hole concentration of 1 and 3 X 1017 cm - 3, respectively, at var- ious temperatures between 20 and 150 K are shown in Figs. 5(b) and 5(c). Only one emission band dominates above 150 K as in the case of the lightly doped sample. When the temperature is lowered to 100 K, these peaks A and B and band C are distinctly separate. As seen in Fig. 5(a), a decrease in the temperature moves the C-band maximum toward shorter wavelength and peak B becomes a wing of this band in the high-energy side. A further decrease in the temperature causes the merging band to dominate the spectrum and then peak A appears as a hump [e.g., 20-K spectrum of Fig. 5(c)] in the short-wavelength wing of this band.

At low temperatures, the shape of the wide main emis- sion peak is strongly asymmetric with a steeper slope on the high-energy side than that on the low-energy side. It

z cn 75 I- z -

J 5 z z si 3 E :

5 (a)

jl0

Mg -doped

‘no 32%e.3 p

p=5x10’6?m-3

77 K

50 K

0 5300 5500 5700 5900 6100 6300 65

WAVELENGTH ( 8)

I

hints that this emission is due to indirect transition.25’26 In addition, we also have investigated that the C-band maxi- mum shifts to higher energies and the FWHM of band C decreases with an increasing excitation level. Therefore, band C has the feature of donor-acceptor (D-A ) pair emis- sion. When the incident excitation density is increased, the more distant recombining pairs saturate and recombina- tion takes place between closer D-A pairs in which the Coulombic attraction is greater, thus the peak of the re- combination emission moves to higher energy. It should be also pointed out that the spectrum of Zn-doped GaP has a band similar to band C observed in our study and it is usually attributed to radiative D-A pair emission.27’28 Bach- rah and Hakki29 and Ermakov et .1.30,31 have demonstrated that the emission transition of band C from the Ini _ ,Ga,P samples with x = 0.64470 is due to D-A pair emission by time-resolved spectroscopy. Ermakov et al. proposed that band C is due to radiative recombination of electrons from residual deep donor levels (such as Te, Si, and S) associated with an indirect X, valley in the direct- band-gap alloys via acceptor levels. At 300 K, the lowest electron state is the donor level at (l,O,O). Since the indi-

5

(b)

r

IIOC

Mg -doped

‘“03PO68P p:lx10’Em-3

A------ 150 K

20 K I

1 5700 5900 6100 6300 6!

1

IO WAVELENGTH ( fi )

FIG. 5. Photoluminescence spectra of Mg-doped In, 32Gao68P layers with p = (a) 5 X 1016, (b) 1 X lo”, and (c) 3 X 10” cm - 3 at various temperatures between 20 and 150 K to show the gradual evolution of the peaks.

4478 J. Appl. Phys., Vol. 71, No. 9, 1 May 1992 Chen, Wu, and Kuo 4478

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n Mg -doped

‘“o.32Goo.sep p = 3xl0”cm‘3

35 K

20K I I 5100 5300 5500 5700 5900 6 IO0 6330 6!

(c) WAVELENGTH (%I

FIG. 5. (Continued.)

rect recombination is slow and the EC - ED separation is small enough so that the electron can reach r. the domi- nant recombination is direct. As the temperature is low- ered, the Tc - X, separation will be decreased from 79 meV at 300 K to 65.5 meV at 77 K (Ref. 32) in the IncJ2Gac6sP alloy but still be direct. The minority carriers excited by photoluminescence into the rc valley either re- combine radiatively or relax to the X, valley. In the X, valley, the carriers are likely to be trapped by donor levels associated with unintentional impurities. This implies that the donor level associated with the X, valley can partici- pate in the emission process even in the direct h32Gao.68P alloy.

It is worth mentioning that band C shifts rapidly to- ward shorter wavelengths at lower temperatures. It is due to the enhancement of the Coulombic interaction term be- tween the Mg acceptors and unidentified deep donors. These phenomena are very similar to the PL spectra of Zn-doped InGaP layers with the same alloy composition, and were discussed in detail in Ref. 18.

IV. CONCLUSIONS

Mg-doped Inc32Gac68P epitaxial layers were grown on ( 100) GaAsc61P0,39 substrates at 800 “C with 10 “C super-

4479 J. Appl. Phys., Vol. 71, No. 9, 1 May 1992

saturation by LPE. The shiny surface morphology and flat interface of the doped layers are the same as those of un- doped layers. The doping effects of Mg on electrical and optical properties have been examined. C-V measurements in Mg-doped layers indicate that the room-temperature hole concentrations from 5 X lOI to 4.2X 10” cm -’ can be obtained with 2.6~ 10 - 5-2X 10 - 4 mole fractions of Mg in the growth solutions. The segregation coefficient of Mis, KM~, is 0.042 at 800 “C. The room-temperature FWHM of the PL peak increases with hole concentration. The acceptor ionization energy of the Mg-doped InGaP layer determined from the 77-K PL spectra varies from 38 to 20 meV as the hole concentration increases from 5 X lOI to 4.2X 10” cm - 3. It shows a linear relationship between the EA value and the cube root of hole concentra- tion. The temperature dependence of photoluminescence from the Mg-doped InGaP layers with three hole concen- trations is also observed. Peak A has the highest photon energy and is due to the band-to-band transition. The rel- ative intensity of peak B increases with the hole concen- tration and, we may attribute the peak to band-to-acceptor recombination. Band C has the feature of donor-acceptor pair recombination and is due to the transition from a deep donor (residual) level associated with indirect X, minima via the Mg acceptor level.

ACKNOWLEDGMENTS

The authors wish to express their thanks to S. T. Hsieh for technical assistance. The study is supported by Na- tional Science Council under Contract No. NSC80,0417- EOO7-12.

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Chen, Wu, and Kuo 4479

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4480 J. Appl. Phys., Vol. 71, No. 9, 1 May 1992 Chen, Wu, and Kuo 4480

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