Liquidphase epitaxial growth and characterization of InGaAsP layersgrown on GaAsP substrates for application to orange lightemittingdiodesChyuanWei Chen and MengChyi Wu Citation: J. Appl. Phys. 77, 905 (1995); doi: 10.1063/1.359017 View online: http://dx.doi.org/10.1063/1.359017 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v77/i2 Published by the American Institute of Physics. Related ArticlesPhoton recycling effect on electroluminescent refrigeration J. Appl. Phys. 111, 014511 (2012) Electrically pumped simultaneous ultraviolet and visible random laser actions from ZnO-CdO interdiffused film Appl. Phys. Lett. 99, 261111 (2011) Giant magneto-light output in three-phase magnetostrictive, piezoelectric, and electroluminescent composites Appl. Phys. Lett. 99, 212503 (2011) Direct evidence for suppression of Auger recombination in GaInAsSbP/InAs mid-infrared light-emitting diodes Appl. Phys. Lett. 99, 141110 (2011) Size-dependent electroluminescence from Si quantum dots embedded in amorphous SiC matrix J. Appl. Phys. 110, 064322 (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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Liquid-phase epitaxial growth and characterizatiqn of InGaAsP layers grown on GaAsP substrates for application to orange light-emitting diodes

Chyuan-Wei Chen and Meng-Chyi Wu Research Institute of Electrical Engineering, National Tsing Hua University, Hsinchu, Taiwan 30043, Republic of China

(Received 30 June 1994; accepted for publication 27 September 1994)

The growth conditions of undoped InGaAsP layers grown on GaA~~rPu,s, substrates and effects of Te and Zn-doping on electrical and optical properties have been examined in detail. The narrowest full widths at half-maximum (FWHM) of photoluminescence (PL) spectra were measured to be 40 meV at 300 K and 12 meV at 8 K for an undoped InGaAsP sample with a background electron concentration of 2X1016 cmm3. Room-temperature carrier concentrations in the range of 1.8X10r7-3.4X101* cmv3 for n-type dopant and 1.6Xl.0f7-2.8X1018 cm ‘a for p-type dopant are obtained reproducibly. The 50 K PL spectra of Zn-doped layers show three distinctive peaks and their relative intensities change with various hole concentrations. Visible light-emitting diodes (LEDs) emitting at 619 nm and employing the InGaP/InGaAsP/InGaP double-heterostructure (DH) grown on a lattice-matched GaAs0.61P0.39 substrate have been fabricated. The DH LEDs are characterized by current-voltage (I-V) measurement, electroluminescence (EL), light output power, and external quantum efficiency. A forward-bias turn-on voltage of 1.8 V with an ideality factor of 1.3 and a breakdown voltage of 16 V are obtained from the I-V measurements. The emission peak wavelength and FWHM of room-temperature EL spectra are around 619 nm and 48 meV (15 nm) at 20 mA, respectively. The light output power of the bare diodes is as high as 0.32 mW at a dc current of 100 mA and an external quantum efficiency of 0.22% is observed. 0 1995 American Institute of Physics.

I. INTRODUCTION Visible-light-emitting sources in the 600 mn wavelength

have attractive potential applications in optical memory, laser printer, outdoor display, and bar-code scanner. Shortening of the emission wavelength is expected to be advantageous for obtaining smaller focused spots and highly luminous light sources for human vision. This has stimulated extensive re- search on the AlGaInP semiconductor alloy system, espe- cially towards the aim of producing shorter wavelengths. The shortest wavelength achieved at present for the well devel- oped AIGaInP/InGaP system lattice matched to GaAs sub- strate is 620-670 nm. *I’ The In, -xGa,AsYP, -Y quaternary alloy is another potential candidate for these applications be- cause of its direct band gap. as high as 2.25 eV (x=0.74, y =O) at 300 K. It can be grown on commercial GaAs,,rPa,s, epitaxial substrates with exact lattice match in the direct band-gap range of 1.88-2.17 eV (660-570 nm).3 Several workers succeeded in the growth of the InGaAsP/InGaAsP double heterostructure (DH) on GaAsu,rPa,,, substrates by liquid-phase epitaxy (LPE) and reported an injection laser operating in the 630-665 nm region with a relatively high threshold current density.4-7 Shima et al.’ reported results on visible dual-wavelength light-emitting diodes (LEDs) using lattice-matched InOJGaa-IP on G~AQ,P,,~ substrates with emission wavelengths of 583 and 652 nm emitted from the InGaP and GaAsP homojunction LEDs. Our previous work9 on InGaP homojunction LEDs grown by LPE operated at 584 nm shows an external quantum efficiency of -0.02%. Recently, some degree of success in the preparation of InGaP LEDs on GaAs or Si substrates using strained-layer- superlattice buffer layers has been demonstrated by several growth techniques.‘0-12 However, they do not exhibit a good device performance.

In this article we present the LPE growth and character- ization of undoped, Te- and Zn-doped Ino~12Gao,ssAso.34Po.66 layers on GaAso,1Po~3g substrates. The fabrication and per- formance of Ino,32Gao~,sP/Ino~,2Gao,ssAso~34Po~~~ DH LEDs emitting in the orange (619 nm) region with a higher external quantum efficiency of 0.22% at 300 K are reported for the first time.

Il. GROWTH AND CHARACTERIZATION OF ~no.~2Gao.~~Aso.s4~o.se LAYERS

InGaAsP epitaxial layers were grown by LPE using a horizontal sliding boat system. The substrates used were GaAso.61Po,3g commercial epitaxial wafers grown by vapor- phase epitaxy on Si-doped (lOO)-oriented GaAs substrate 2” off toward (110) via a -30-pm-thick graded composition of GaAsP layer which alleviates large lattice mismatch and helps threading dislocations escaped in the horizontal direc- tion. The surface GaAs,,rPo~,, layer is 40 pm thick and n-doped 7X1016 cm-3 with Te. For the LPE growth of InGaAsP, the melt contained -3 g of In as the solvent and undoped polycrystalline GaAs, InAs, and GaP as sources of Ga, As, and P. The In melt was baked at 900 “C for 10 h under a purified H2 ambient to reduce the background carrier concentration in the epitaxial layers. After the baking pro- cess, appropriate polycrystalline solutes of-GaAs, InAs, and Gap, and dopants of Te or Zn were added to form the Ino.~45Gao.o3~Aso.lO6P0.011 growth solution. r3 The epitaxial layers were grown at 795 “C with a cooling rate. of 0.3 “Cl min and had a typical thickness of 3 ,um during a fixed growth period’ of 5 min.

For all the undoped and doped InGaAsP samples, the surface morphologies are very flat and mirror-like, but with a

J. Appl. Phys. 77 (2), 15 January 1995 QO21-8979/95/77(2)/905/5/$6.00 0 1995 American Institute of Physics 905

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g 2 E 5 c 55 i5 2 w 2 8 ii 5 3 P 8 (L

undoped

InGaAsP

20K

570 590 610 630 650

WAVELENGTH (nm)

PIG. 1. Photoluminescence spectra of undoped In,lzGao,ssi\so34P,.a~ layers with a background concentration of 2X1016 cmm3 at various temperatures between 8 and 150 K to show the gradual evolution of the peaks.

so-called cross-hatched-pattern surface, which is inherited from the original misfit dislocations introduced during the graded layer growth of GaAsP on the GaAs substrate.13 The interface between the epitaxial layer and the GaA~e.~rPu.~~ substrate is also flat and free from inclusions. The lattice mismatch between the epitaxial layer and GaAsP substrate normal to the wafer surface, determined by the (400) reflec- tion planes of a double-crystal x-ray diffraction spectra, is controlled within the range O-+0.2%. The electrical and op- tical properties of the InGaAsP layers were evaluated by the mercury-probe capacitance-voltage and photoluminescence (PL) measurements. Details of the growth- conditions and characterization techniques are given elsewhere.‘4’15

Baking of the In-rich melt at 900 “C for 10 h produced a typical background electron concentration of around 2X 1Or6 cme3 at 300 K. Figure 1 shows the temperature dependence (T=8-150 K) of the emission spectra of a typical undoped film measured with the identical excitation density of 0.5 W/cmz. All the PL curves are normalized to the same main peak intensity. The 8 K PL spectrum consists of a stronger emission peak and a weaker band (denoted B and G) located at 2.089 eV (593.5 nm) and 1.973 eV (628 nm), respectively. The band labeled G is related to the radiation emission via the near-band-gap transition of the GaA~o.,rPa,~ substrate

Te - or Zn - doped I no ,eGs,,A~,s,Po 66

- Te-doped _

- h-doped

lo%----~~-~ s ,,,..*’ IO5 IO-

, o.3 -----p--J 10-I

Te OR Zn MOLE FRACTION IN GROWTH SOLUTION,

X+, OR %f.

FIG. 2. The mom-temperature carrier concentrations (n and p) of Te- and %-doped Ina12Gaa.8sAsa,34Po.65 layers grown on GaA~a.~rPa,~~ substrate as a function of Te and Zn mole fractions in growth solutions, X& and X&,, respectively.

surface layerr and will not be discussed further in this ar- ticle. Peak B dominates the low-temperature PL spectra and remains at the constant position in the temperature range 8-32 K. At 35 K, a new peak A located at 2.093 eV (592.4 nm) in the short-wavelength side of peak B emerges and the emission of peak B disappears. A further increase in the tem- perature moves the A-peak emission toward longer wave- lengths and broadens its full width at half-maximum (FWHM). In addition, the intensity of peak A increases lin- early with incident power density while that of peak B will saturate at higher laser powers for the low-temperature PL spectra (not shown in Fig. 1). Peak B is displaced by 4 meV below peak A. The normally encountered shallow donors have an ionization energy less than 4 meV and the ionization energy of donors (E,) is usually smaller than that of accep- tor (EA).‘7 Therefore, peaks A and B can be attributed to the near-band-to-band and donor-to-valence-band transitions, re- spectively. The binding energy of the unidentified residual donor in the undoped In,l2G~.ssAs.,,P,.66 layers is 4 meV The narrowest PL FWHMs are 12 meV at 8 K and 40 meV at 300 K, which are much better than those reported previously.13T’*

Figure 2 shows the room-temperature carrier concentra- tions (n and p) of Te- and Zn-doped Ino.lzGao.ssAs,3,P,.6~ layers grown on G~As,,,,P,,~ substrates versus Te and Zn mole fractions in the growth solutions, X& and X& , respec- tively. The carrier concentrations increase linearly above 10” cmS3 as both dopant mole fractions in the solutions are increased. For the Te dopant, the electron concentration in- creases linearly from 1.8X1017 to 3.2X10” cmS3 as-X& var- ies from 9.8X10U4 to 1.1X10-’ mole fraction. When Xie is further increased, the electron concentration saturates at a value near 3.4X 10” cm-3. On the other hand, the hole con- centration also increases linearly from 1.6X10i7 to 2.7X10” cme3 as X& varies from 5.4X10b5 to 8.1 X10e4 mole frac- tion, and toward saturation at Xkn greater than 8.1X10e4. The segregation coefficients k in In0,1zGae&&rsJ~P0,66 epi-

906 J. Appl. Phys., Vol. 77, No. 2, 15 January 1995 C.-W. Chen and M.-C. Wu

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F ‘5 m 5 c z 25 ki ti z x i 5 3 e P CL

Te-doped

‘“0.,2Ga.a~%.34P,.66

,=7x10’hi3

8K 300K

594.5 nm 622 nm

WAVELENGTH (nm )

FIG. 3. 300 and 8 K photoluminescence spectra of a Te-doped In,,IzGa0,88As034P0,66 layer with an electron concentration of 7X10” cm- 3. The intensities are normalized to the same value.

g 3 ii 9 g 55 B ii 8 zl 0 9 5 3 e 0 E 5 ul

taxial layers grown at 795 “C are estimated as 0.0043 for the Te dopant and 0.07 for the Zn dopant, assuming that all dopants are ionized.

Typical 300 and 8 K PL spectra, which are normalized to the same peak intensity, of a Te-doped &,,,GQ layer with an electrbn concentration of 7x10 i?

sAsO 34P,,,6 cm -3 are

shown in Fig. 3. The only room-temperature emission band of the Te-doped IIlo.IzGa,~sAs,,,P,,, layer is due to the re- combination of free electrons and free holes, as seen from the peak energy and spectrum shape of the emission band due to free-electron-to-free-hole transition in the undoped layer. The 8 K PL spectrum also consists of only one emis- sion band with a FWHM of 57 meV. The PL peak wave- lengths at 300 and 8 K of the Te-doped Ino.12GaassAso.34Pn.66 layers are located at 622 and 594.5 nm which are slightly larger than those of the undoped layers at 620 (peak A) and 593.5 (peak B in the low-temperature PL spectra) nm, re- spectively. This indicates that the donor ionization energy of Te is very small (<7 meV) and tends to merge with the residual donor states and the conduction band states.

The 50 K PL spectra excited by a power of 0.5 W/cm2 for various hole concentrations are presented in Fig. 4. These spectra are also normalized to the same main peak intensity. The PL spectrum of the undoped sample, as shown in curve (a) of Fig. 4, is used as a referenbe. For the sample doped with a hole concentration of 1.6X1017 cm-j, in addition to peak A, the PL spectrum exhibits a peak C located at 2.058 eV (602 mnj. As shown in curve (c) of Fig. 4 with increasing hole concentration to p=2.5X10’7 cmw3, the intensity of peak C grows and a new broad band E located at 2.051 eV (604.5 nm) emerges. When the hole concentration is in- creased to above 6X 1017 cmm3, peak A disappears and the 50

J. Appl. Phys., Vol. 77, No. 2, 15 January 1995 C.-W. Chen and M.-C. Wu 907

Zn-doped

hJ.,2Gaa~a,~P,.,

50K PL

(4 undoped

n=2xld%d3

570 590 610 630 650 670 690

WAVELENGTH (nm)

FIG. 4. 50 K PL spectra of Zn-doped In,,,G~,ssAs,,,P,,~~ layers with a carrier concentration of (a) n=2X1016 (undoped), (b) p=1.6X101’, (c) 25X10’7, (d) ~XICI’~, and (e) 1.5X10” cmm3. These spectra are also nor- malized to the same main peak intensity.

K PL spectrum is dominated by peak C with a FWHM of 40 meV. Peak C is located at 19-31 meV below peak A and its relative intensity increases with hole concentration, which confirms that peak C is definitely associated with the transi- tion from the conduction band to the acceptor level of Zn. The acceptor ionization energy of Zn, E, , is calculated as 19-31 meV. Peak E, displaced by -60 meV below peak A, shifts to higher energies with increasing excitation level. Be- sides, it also shifts to longer wavelengths and quenches rap- idly compared with the emission of peaks A and B at higher temperatures (not shown in Fig. 4).‘” Therefore, peak E has the feature of a donor-acceptor pair emission. It should be pointed out that the PL spectrum of Z&doped GaP has a band with the similar feature and its is attributed to radiative deep-donor-to-acceptor recombination.“’ Accordingly, the ionization energy of the residual deep donor in the Zn-doped Ino~,2Ga0,88AS0~34P0,66 layers is estimated to -38 meV.

Figure 5 shows the quantities E, determined at 50 K as a function of p 1’3 for the Zn-doped Ino~,,Gao~s,Aso,34Po,~~ layers. It shows a linear decrease of the acceptor ionization energy as the cube root of the hole concentration and can be

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3 E

i

Zn -doped In

,4 35 o.12Gao.***so.34 po.66

p”3(cm-’ )

FIG. 5. Acceptor ionization energy determined at 50 K in Zn-doped lrb,12Ga,s~s,,,,P,,,, plotted as a function of the cube root of the room- temperature hole concentration.

represented by the following empirical expression?

where Ei is the ionization energy for infinite dilution and Q is a constant. Calculated values for Ei and (Y are 44.4 meV and 2.4X10p5 meV cm, respectively. ‘The E, value of Zn varies from 31 to 19 meV as p varies from 1.6X10r7 to 1.5X 10” cme3.

ill. FABRICATION AND PERFORMANCE OF InGaP/ InGaAsP/lnGaP DH LEDs

The InGaP/InGaAsP/InGaP double heterostructure was grown by LPE on the Te-doped GaAs,,~rP,,s, substrate. The DH is composed of a 1 pm Ino,12Ga,.ssAs,,,P,.~~ active layer bounded on the top by a 4 ,um Zn-doped In,,a,Ga,,,sP clad- ding layer (p=lXIO1* cmm3) and on the bottom by a 3 pm Te-doped In,,,,Ga,,,,P cladding layer (n =2X 101* cmW3). The Ino.lzGao.ssAso.34Po.66 active layer was grown at 795 “C with a cooling rate of 0.3 “C/min for 3 min. The Te- and Zn-doped Inas,Ga0,~8P layers were grown at 796 and 794 “C for 5 .and 6 min at a cooling rate of 0.2 “C mm, respectively, as described previously.r5

After the growth process, the substrate was thinned to about 250 pm in order to reduce the bulk resistance and remove the contamination from evaporated phosphorous dur- ing growth. After wafer cleaning, a AulAuGeNiiNi structure with layer thickness of 200/100/10 nm was evaporated on the substrate side to form the IZ contact. For the p contact, a 100 nm layer of AuBe was vacuum deposited onto the p-InGaP surface and patterned into lOO-pm-diam dots using a 10 g RI: 1 g Iz: 100 cc Ha0 etching solution to allow light emis- sion through the top of the die. After deposition and pattern definition, the wafer was alloyed at 420 “C for 5 min in a hydrogen ambient to obtain good ohmic contact. The wafer

-2 ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ : ’ ’ -20 16 -12 a 4 0 4

VOLTAGE (V)

FIG. 6. Typical current-voltage characteristics of the InGaP/InGaAsP/InGaP orange DH LEDs.

was next sawed into the die dimensions of 300 pxn~300 pm. The chip was then attached and bonded to a TO-5 pack- age for measurements.

A typical current-voltage (I-V) characteristics of the or- ange DH LEDs is shown in Fig. 6. It exhibits a forward-bias turn-on voltage of 1.8 V with an ideality factor of 1.3. In the reverse direction, the breakdown voltage of 16 V is observed. Room-temperature electroluminescence (EL) emission spec- tra at different forward currents of 5, 10, 20, 40, and 60 mA of bare orange DH LEDs are shown in Fig. 7. All the EL emission peak wavelengths are located at around 620 mn and the peak wavelength shifts to slightly longer wavelengths with increasing injection current. This is due to the band-gap shrinkage caused by Joule heating. The FWHM value at 20

~~~~~..~~..~ Homojunction IEDS

I

570 590 610 630 650 670

WAVELENGTH (nm)

FIG. 7. Room-temperature EL emission spectra with forward currents of 5, 10, 20, 40, and 60 mA of bare orange InGaP/InGaAsP/InGaP DH LEDs. The dotted EL spectrum as shown in this figure is for the EL spectrum of a’ bare Ino,12G~,ssAso,34Pa.66 homojunction LED at 20 mA.

908 J. Appl. Phys., Vol. 77, No. 2, 15 January 1995 C.-W. Chen and M.-C. Wu

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0.3

z a

5 0.2

2 i3

0

CURRENT (mA)

FIG. 8. Light output power measured at room temperature as a function of dc current for the bare DH LEDs.

mA of the orange LED is around 48 meV (15 nm) -which is comparable to that of 48 meV (16 nm) for the AlGaAs/IuGaP red LEDs at 660 nm22 and 55 meV (14.5 nm) for InGaP yellow-green LEDs at 584 nm? The EL spectrum of Ino.12G~,,,As,,,P,.~~ p-n homojunction LEDs at 20 mA is also denoted in Fig. 7 by the dotted curve. The FWHM of the homojunction LEDs is around 53 meV (17 nm) at 20 mA which is larger than that of DH LEDs and its emission inten- sity is about two orders in magnitude lower than that of the DH LEDs.

Figure 8 shows the light output power measured at room temperature versus dc current characteristics of the bare or- ange DH LEDs. It can be seen that the DH LEDs exhibit a linear increase of light output power as increasing injection current. An output power of 0.32 m W can be achieved at a current of 100 mA, corresponding to a current density of 1.27 kA/cm2 for these devices with a lOO-pm-diam contact. While the output power is saturated at 120 mA and becomes degraded with further increasing injection current. Joule heating, carrier leakage, and nonradiative recombination from the active region are of importance in causing this saturation.= The external quantum efficiency at 20 mA for the DH LEDs achieved is about 0.22%.

IV. CONCLUSIONS

The electrical and optical properties of undoped, Te-, and Zn-doped Inn.12Gao.88Aso.34P,.~~ layers have been exam- ined. The undoped layers always give n-type conduction with a background electron concentration of 2X10r6 cme3. The carrier concentration for Te- and Zn-doped I~.~zG~.ss&a.s~Pa.s~ can be obtained as high as 3.4X101*

and 2.8X10” cme3, respectively. The segregation coeffi- cients of Te and-in in InGaAsP are 0.0043 and 0.07 at 795 “C, respectively. The variations of 50 K PL spectra with various hole concentrations in the Zn-doped InGaAsP layers are described. The acceptor ionization energy determined at 50 K varies from 31 to 19 meV as hole concentration in- creases from 1.6X10r7 to 1.5X101* cme3. I~ has a linear relationship with the cube root of hole concentration P”~. Finally, we have demonstrated the feasibility of growing ~~o.,,~~o.6s~/Irb.l,~~o.8s~~o.~~~o.66~~o.~~~~o.68~ orange DH LEDs on GaAso.,,Po,,, substrates by LPE. The InGaP/ InGaAsP/InGaP orange DH LEDs were fabricated and ex- hibit a forward-bias turn-on voltage of 1.8 V with an ideality factor of 1.3 and a breakdown voltage of 16 V. The EL spec- tra have the emission peak wavelength and FWHM around 620 run and 48 meV (15 nm) at 20 mA, respectively. The external quantum efficiency of 0.22% has been achieved for the bare DH LEDs.

ACKNOWLEDGMENTS

The authors wish to express their deep gratitude to Y H. Yeh for his technical assistance. This study is supported by National Science Council under Contract No. NSC 82-0417- E-007-239.

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