Journal of Crystal Growth 189/190 (1998) 496—499

Temperature dependence of photoluminescenceof GaP

1~xN

xalloys

Hiroyuki Yaguchi!,*, Goshi Biwa!, Seiro Miyoshi!, Daiichiro Aoki!, Keisuke Arimoto!,Kentaro Onabe!, Ryoichi Ito!, Yasuhiro Shiraki"

! Department of Applied Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan" Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153, Japan

Abstract

We report on the temperature dependence of photoluminescence (PL) in GaP1~x

Nx

alloys. With increasing temper-ature the feature of the PL spectrum changes considerably and the luminescence with lower energies becomespredominant. This is explained by thermal activation processes from localized states to the state where excitons can movefreely. By estimating the activation energies from the temperature dependence of the PL intensity, it is found that the statewhere excitons can move freely is formed below the band edge of GaP and shifts to lower energies with increasingnitrogen content. This state is considered to originate from the isoelectronic trap due to isolated nitrogens in GaP (A line)and correspond to the band edge of the GaP

1~xN

xalloy system. ( 1998 Elsevier Science B.V. All rights reserved.

PACS: 78.55.C; 81.05.E

Keywords: GaPN; Band gap; Photoluminescence

1. Introduction

In the GaP1~x

Nxalloy system, two components,

GaP and GaN, are different in the crystal structure(GaP: zincblende, GaN: wurtzite) and in the latticeparameter. Owing to these differences there is alarge miscibility gap which leads to the difficulty in

*Corresponding author. Fax: #81 3 3481 4509; e-mail:yaguchi@photonics.rcast.u-tokyo.ac.jp.

growing high-quality epitaxial GaP1~x

Nx

alloyfilms. However, there have recently been severalworks concerning epitaxial growth of GaP

1~xN

xalloys by metalorganic vapor-phase epitaxy [1]and molecular beam epitaxy [2—4], which are thegrowth techniques under nonequilibrium condi-tions. Since the GaP

1~xN

xalloy consists of two

wide-band gap semiconductors, GaP (2.3 eV) andGaN (3.4 eV), it is one of the candidates for light-emitting materials with a spectral range from greento ultraviolet. Although the band gap energy ofGaP

1~xN

xalloy is expected to simply increase with

0022-0248/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved.PII S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 3 3 8 - 8

increasing N content, in fact the PL peak shifts tolower energies [1,2]. This indicates an extremelylarge band gap bowing in the GaP

1~xN

xalloy. The

large band gap bowing is considered to be at-tributed to the large difference in electronegativitybetween N and P.

In this study, we have investigated the temper-ature dependence of the PL intensity of GaP

1~xN

xalloys with various N content in detail to examinethe band gap bowing in the GaP

1~xN

xalloys. We

found that the state where excitons can move freelyis formed below the band edge of GaP and shifts tolower energies with increasing nitrogen content.This state is considered to originate from theisoelectronic trap due to isolated nitrogens in GaP(A line) and corresponds to the band edge of theGaP

1~xN

xalloy system.

2. Experimental procedure

The samples used in this study were grown onGaP(1 0 0) substrates by low pressure (60 Torr)metalorganic vapor-phase epitaxy (MOVPE).Trimethylgallium, phosphine and 1,1-dimethyl-hydrazine were used as Ga, P and N sources,respectively. Details about the MOVPE growthwas described elsewhere [5]. The substrate temper-ature was 630—700°C. X-ray diffraction was used todetermine the nitrogen content assuming Vegard’slaw between the nitrogen content and the latticeconstant of GaP

1~xN

xalloys. In spite of the large

miscibility gap in this alloy system high-qualitysamples with nitrogen contents up to 3.2% wereobtained due to the adoption of dimethylhydrazine.Photoluminescence (PL) was performed at temper-atures from 6 to 200 K. A He—Cd laser (325 nm)was used as the excitation source for PL measure-ments.

3. Results and discussion

Fig. 1 shows the temperature dependence of thePL spectrum of the GaP

1~xN

xalloy with nitrogen

content of 0.228%. At low temperatures, sharp linesdue to nitrogen pairs (NN

1and NN

3) and their

phonon replicas are clearly observed. With increas-

Fig. 1. Temperature dependence of photoluminescence spec-trum of GaP

1~xN

x(x"0.228%) alloy.

ing temperature, the feature of the PL spectrumchanges considerably and the luminescence withlower energies becomes predominant. Temperaturedependence of the PL intensity of NN

1and NN

3lines in GaP

1~xN

x(x"0.228%) is shown in Fig. 2.

The intensity of each line rapidly decreases withincreasing temperature. This can be explained bythermal activation processes from localized statesto nonradiative recombination centers through thestates where excitons can move freely, as schemati-cally shown in Fig. 3. The nonradiative recombina-tion centers are mainly due to misfit dislocationscaused by the lattice mismatch between theGaP

1~xN

xepitaxial layer and the GaP substrate

[6]. The “mobile state” is defined as the energy levelwhere excitons can move around freely with noactivation energies. On the basis of this model,temperature dependence of the PL intensity is

H. Yaguchi et al. / Journal of Crystal Growth 189/190 (1998) 496–499 497

Fig. 2. Temperature dependence of photoluminescence inten-sity of NN

1and NN

3lines in GaP

1~xN

x(x"0.228%) alloy.

Fig. 3. Schematic diagram of thermal activation process.

expressed as

I(¹)"I0

1#C exp(!E!/k¹)

, (1)

where I0, E

!and ¹ are the PL intensity at 0 K, the

activation energy and temperature, respectively.The parameter C is dependent on the radiative andnonradiative recombination lifetime. Accordingto this model, the energy level obtained by addingthe activation energy to the luminescence energyshould correspond to the “mobile state”. However,excitons activated to the “mobile state” partly fallinto some localized radiative levels again. As a re-sult of this process, the activation energy may beoverestimated, especially for lower energy levels.

Fig. 4. Determination of “mobile state” by extrapolating theline to E

!"0.

For this reason, we determined the “mobile state”by extrapolating the line to E

!"0, as shown in

Fig. 4. In fact, the activation energies for NN1

andNN

3lines are estimated at 152 and 22 meV, respec-

tively and overestimated as can be seen from thisfigure.

The “mobile state” is found to be lower in energythan the free exciton level in GaP. For comparisonwe also show the absorption edge determined fromphotoluminescence excitation [7] in Fig. 4. The“mobile state” is close to the absorption edge butlocated higher in energy than the absorption edge.The “mobile state” can be successfully determinedby such extrapolation for GaP

1~xN

xalloys with

various nitrogen contents.Fig. 5 shows the “mobile state” determined

based on the temperature dependence of the PLintensity and the absorption edge determined fromphotoluminescence excitation measurements [7].Both the “mobile state” and the absorption edgeshift to lower energies with increasing nitrogen con-tent and appear to approach the A line when xP0.This indicates that the “mobile state” as well as theabsorption edge originates from the band forma-tion of the A line [7]. However, at lower nitrogencontents (x(0.05%), nitrogen atoms are isolatedfrom each other and excitons trapped by the iso-lated nitrogen atoms cannot move around freely.This is also supported by a theoretical study usingtight-binding approximation [8]. In this region,thus, the “mobile state” coincides with the free

498 H. Yaguchi et al. / Journal of Crystal Growth 189/190 (1998) 496–499

Fig. 5. “Mobile state” and absorption edge of GaP1~x

Nx

alloys.

exciton level in GaP. In addition, the energy differ-ences between the “mobile state” and the absorp-tion edge shown in Fig. 5 are considered to be dueto the contribution from localized levels becausethe absorption by the localized levels is little butcannot be neglected. In this way, the “mobile state”determined in this study is rather similar to theband edge in other alloy systems, such as AlGaAsor GaAsP [9].

4. Conclusions

We have studied the temperature dependence ofphotoluminescence (PL) in GaP

1~xN

xalloys. With

increasing temperature the feature of the PL spec-trum considerably changed and the luminescencewith lower energies became predominant. This isexplained in terms of thermal activation processesfrom localized states to the states where excitonscan move freely. By estimating the activationenergies based on the temperature dependence ofthe PL intensity, it is found that the stateswhere excitons can move freely are formed belowthe band edge of GaP and shift to lower energieswith increasing nitrogen content. These statesare considered to originate from the A lineand correspond to the band edge of this alloysystem.

Acknowledgement

We would like to thank S. Otake for his technicalsupport.

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