- Email: [email protected]
1.55 μm Er-doped GaN LED
Solid-State Electronics 43 (1999) 1231±1234
1.55 mm Er-doped GaN LED H. Shen a,*, J. Pamulapati a, M. Taysing a, M.C. Wood a, R.T. Lareau a, M.H. Ervin a, J.D. Mackenzie b, C.R. Abernathy b, S.J. Pearton b, F. Ren b, J.M. Zavada c a
US Army Research Laboratory, Adelphi, MD 20783-1197, USA b University of Florida, Gainesville, FL 32611, USA c US Army Research Oce, Research Triangle Park, NC 27709, USA Received 19 October 1998; received in revised form 15 January 1999; accepted 7 February 1999
Abstract Erbium (Er) doped semiconductors are of interest for light-emitting device applications operating at around 1.55 mm and for the potential integration with other semiconductor devices. However, the optical emission of Er3+ ions in semiconductors has not been as ecient as in dielectric materials, particularly at room temperature. This may be because ionic bonds, which are characteristic of dielectrics, are better suited for forming the required Er3+ energy levels than are covalent bonds, which are present in most III-V semiconductors. In this paper, we report 1.55 mm emission from an Er-doped GaN LED. We also discuss the eect of the measurement temperature on the emission spectrum as well as the eect of sample annealing on the emission spectrum. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction Rare-earth-doped semiconductors have sparked considerable interest for the following: ®rst, to exploit the properties that rare-earth elements have exhibited in dielectrics, and second, for potential integration of the rare-earth elements with existent semiconductor devices. Particular emphasis has been placed on doping crystalline silicon with Erbium (Er) solely for the purpose of creating light-emitting devices operating at around 1.55 mm in silicon. However, the optical emission of Er3+ ions in semiconductors is not nearly as ecient as in dielectric materials, particularly at room temperature [1,2]. The lower eciency could be
* Corresponding author. Tel.: +1-301-394-1531; fax: +1301-394-1746. E-mail address: [email protected] (H. Shen)
because the ionic bonds, which are characteristic of dielectrics, are better suited for forming the required Er3+ energy levels than are the covalent bonds present in most III-V semiconductors. Recently, Wilson et al. [3] doped group III-nitride semiconductor thin ®lms with Er atoms and observed strong optical emission from the Er3+ ions at room temperature under optical excitation. In this paper, we report 1.55 mm emission from an Er-doped GaN LED. We also discuss the eect of the measurement temperature on the emission spectrum as well as the eect of the sample annealing on the emission spectrum. 2. Experimental The samples used in this study were grown on ptype silicon wafers by metal-organic molecular beam epitaxy (MOMBE). Details of the growth parameters
0038-1101/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 8 - 1 1 0 1 ( 9 9 ) 0 0 0 5 6 - 8
1232
H. Shen et al. / Solid-State Electronics 43 (1999) 1231±1234
Fig. 1. SIMS pro®le.
can be found elsewhere [4]. The Er was co-doped at the time of growth with the use of a shuttered eusion oven. The GaN:Er layer was grown on top of a 300 nm undoped GaN buer layer. Subsequent to growth, 500-mm diameter diodes were processed by dry etching. Ti/Pt/Au (200 AÊ/400 AÊ/2500 AÊ) based metallization was deposited through a shadow mask on the asgrown GaN to avoid any contamination. Then, the back of the Si substrate was polished to reduce the light scattering. To improve the contact resistance, an anneal was performed in an AG 410 Rapid Thermal Anneal (RTA) system at 4008C for 1 min under He ambient. Chemical analysis and acquisition of depth pro®le information was performed with a Physical Electronics Inc. (PHI) 6300 quadrupole based secondary ion mass spectrometer, using an O primary beam 70% ion-optical linear gating, a high mass resolution magnetic sector Cameca IMS-6F secondary ion mass spectrometer, using Cs as the primary species. A Hitachi S-4500 scanning electron microscope (SEM) equipped with a Princeton Gamma Tech energy dispersive X-ray analyzer (EDX) was employed to perform SEM and energy dispersive measurements respectively on the sample. The depth of the analysis craters was measured with a Tencor Alpha-Step 300 pro®ler. Electroluminescence (EL) measurements were performed by biasing the diode with a pulse generator at a frequency of 400 Hz and a 10% duty cycle. Low
temperature measurements were performed by mounting the silicon side of the sample to a brass holder in a closed-cycle helium refrigerator. Mounting the sample in this manner ensures good thermal contact and good heat dissipation. The brass holder has a small hole that allows the emission through the silicon substrate to be collected from the back side. Emitted light was analyzed through a 1 m monochromator and detected using a liquid nitrogen cooled Ge detector and standard lock-in techniques.
3. Results and discussion SEM depth pro®le analysis is shown from the quadrupole based SIMS measurement in Fig. 1. This ®gure shows that the Er distribution in the GaN:Er epilayer is uniform. The sample shows a depth structure as measured by SIMS of 1428 nm thick GaN:Er layer on 374 nm of GaN on a Si substrate. The Er levels in the sample were determined by EDX to be 0.85% using standardless quantitation methods. The decay length (84±16% matrix intensity) of the Er signal at the GaN:Er/GaN interface is 38 nm The decay length of the Ga signal at the GaN/Si interface is 25 nm. The Si signal increases from 16 to 84% of the peak intensity at 25 nm. The Si 28 AMU species was followed. The apparent presence of Si within the GaN layer is likely
H. Shen et al. / Solid-State Electronics 43 (1999) 1231±1234
Fig. 2. Room temperature (300 K) electroluminescence from the Er doped GaN diodes after electrical current heating (dashed line) and after thermal annealing (solid line).
to be due to the unresolved detection of the N+ 2 species. Before annealing, the sample exhibits a breakdown
Fig. 3. EL spectra taken from 25±300 K. The current density is held constant at 5 A/cm2.
1233
Fig. 4. Integrated intensity as a function of temperature.
at very high voltages, 050 V. Prior to thermal annealing, though, no emission is evident in either bias direction. However, we noticed that in one diode, after current induced heating (08 W), electroluminescence appeared at 1.54 and 1.35 mm (dashed line in Fig. 2). Subsequently, the samples were annealed in an RTA to form a better alloy to the GaN material. After annealing, all the diodes emitted light when reverse biased beyond 10 V. The devices though exhibited a softer breakdown at 10 V. The solid line of Fig. 2 shows the EL spectra taken for the diode samples under reverse bias conditions after annealing. No emission is observable under forward biased operation. Fig. 3 shows EL spectra taken from 25±300 K with a constant current density of 5 A/cm2. Spectra from the dierent temperatures have similar line-shapes. This broad emission is known to occur in reverse biased junctions [5]. We attribute the 1.54 mm emission to the Er3+ emission. At lower temperatures this feature shifts slightly towards a shorter wavelength (1.53 mm). The origin of the shorter wavelength feature is still undetermined. In Er doped silicon, this feature was attributed to defect bands found in ion-damaged silicon after annealing [6] but in our samples, the silicon substrate is not Er doped; nor is the sample ion implanted, so this is probably not the case. Fig. 4 shows the temperature dependence of the integrated EL intensity. The room temperature intensity is about 12.5% of the 30 K value. The Er doped GaN exhibited bright EL even at room temperature. The
1234
H. Shen et al. / Solid-State Electronics 43 (1999) 1231±1234
4. Conclusions Priolo et al. [8] have studied the temperature dependence of the EL from Er doped silicon. They also observed strong 1.54 mm emission under reverse bias. The quenching of the EL signal, as a function of increasing temperature, is a factor of 8. In general the optical emission of Er3+ ions in semiconductors has not been as ecient as it is in dielectric materials. It is believed that wide band-gap semiconductors serving as a host material should lead to a weaker thermal quenching. This is due to the fact that more ionic bonds are better for forming the required Er3+ energy levels. Our results indicate that GaN EL can exhibit much stronger quenching compared to Er doped silicon. References Fig. 5. Integrated intensity as a function of applied current.
[1] Mears PJ, Reekie L, Jauncey IM, Payne DN. Electron Lett 1987;23:1026.
fact that the emission at room temperature is quenched by 087%, when compared to the 30 K value, indicates that the GaN host material somewhat suppresses the temperature quenching of the Er3+ EL signal. The amount of quenching that we observe is still large compared to previous observations [7] but those samples were explored using below bandgap excitation. During EL, the material is being excited by energies above the bandgap. Fig. 5 shows the dependence of the integrated EL intensity on excitation current. The measurement was performed at room temperature. A threshold for emission was clearly observed, indicating that the mechanism for the Er3+ emission is impact ionization by hot carriers [5].
[2] Desurvire E, Simpson RJ, Becker PC. Optic Lett 1987;12:888. [3] Wilson RG, Schwartz RN, Abernathy CR, Pearton SJ, Newman N, Rubin M, Fu T, Zavada JM. Appl Phys Lett 1994;65:992. [4] Mackenzie JD, Abernathy CR, Pearton SJ, HoÈmmerich U, Wu X, Schwartz RN, Wilson RG, Zavada JM. J Cryst Growth 1997;175/176:84. [5] Polman A. J Appl Phys 1997;82:1. [6] Davies G. Phys Rep 1989;176:83. [7] Thaik M, HoÈmmerich U, Schwartz RN, Wilson RG, Zavada JM. Appl Phys Lett 1997;71:2641. [8] Priolo F, Coa S, Franzo G, Polman A. Mat Res Soc Symp Proc 1996;422:305.
1.55 mm Er-doped GaN LED H. Shen a,*, J. Pamulapati a, M. Taysing a, M.C. Wood a, R.T. Lareau a, M.H. Ervin a, J.D. Mackenzie b, C.R. Abernathy b, S.J. Pearton b, F. Ren b, J.M. Zavada c a
US Army Research Laboratory, Adelphi, MD 20783-1197, USA b University of Florida, Gainesville, FL 32611, USA c US Army Research Oce, Research Triangle Park, NC 27709, USA Received 19 October 1998; received in revised form 15 January 1999; accepted 7 February 1999
Abstract Erbium (Er) doped semiconductors are of interest for light-emitting device applications operating at around 1.55 mm and for the potential integration with other semiconductor devices. However, the optical emission of Er3+ ions in semiconductors has not been as ecient as in dielectric materials, particularly at room temperature. This may be because ionic bonds, which are characteristic of dielectrics, are better suited for forming the required Er3+ energy levels than are covalent bonds, which are present in most III-V semiconductors. In this paper, we report 1.55 mm emission from an Er-doped GaN LED. We also discuss the eect of the measurement temperature on the emission spectrum as well as the eect of sample annealing on the emission spectrum. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction Rare-earth-doped semiconductors have sparked considerable interest for the following: ®rst, to exploit the properties that rare-earth elements have exhibited in dielectrics, and second, for potential integration of the rare-earth elements with existent semiconductor devices. Particular emphasis has been placed on doping crystalline silicon with Erbium (Er) solely for the purpose of creating light-emitting devices operating at around 1.55 mm in silicon. However, the optical emission of Er3+ ions in semiconductors is not nearly as ecient as in dielectric materials, particularly at room temperature [1,2]. The lower eciency could be
* Corresponding author. Tel.: +1-301-394-1531; fax: +1301-394-1746. E-mail address: [email protected] (H. Shen)
because the ionic bonds, which are characteristic of dielectrics, are better suited for forming the required Er3+ energy levels than are the covalent bonds present in most III-V semiconductors. Recently, Wilson et al. [3] doped group III-nitride semiconductor thin ®lms with Er atoms and observed strong optical emission from the Er3+ ions at room temperature under optical excitation. In this paper, we report 1.55 mm emission from an Er-doped GaN LED. We also discuss the eect of the measurement temperature on the emission spectrum as well as the eect of the sample annealing on the emission spectrum. 2. Experimental The samples used in this study were grown on ptype silicon wafers by metal-organic molecular beam epitaxy (MOMBE). Details of the growth parameters
0038-1101/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 8 - 1 1 0 1 ( 9 9 ) 0 0 0 5 6 - 8
1232
H. Shen et al. / Solid-State Electronics 43 (1999) 1231±1234
Fig. 1. SIMS pro®le.
can be found elsewhere [4]. The Er was co-doped at the time of growth with the use of a shuttered eusion oven. The GaN:Er layer was grown on top of a 300 nm undoped GaN buer layer. Subsequent to growth, 500-mm diameter diodes were processed by dry etching. Ti/Pt/Au (200 AÊ/400 AÊ/2500 AÊ) based metallization was deposited through a shadow mask on the asgrown GaN to avoid any contamination. Then, the back of the Si substrate was polished to reduce the light scattering. To improve the contact resistance, an anneal was performed in an AG 410 Rapid Thermal Anneal (RTA) system at 4008C for 1 min under He ambient. Chemical analysis and acquisition of depth pro®le information was performed with a Physical Electronics Inc. (PHI) 6300 quadrupole based secondary ion mass spectrometer, using an O primary beam 70% ion-optical linear gating, a high mass resolution magnetic sector Cameca IMS-6F secondary ion mass spectrometer, using Cs as the primary species. A Hitachi S-4500 scanning electron microscope (SEM) equipped with a Princeton Gamma Tech energy dispersive X-ray analyzer (EDX) was employed to perform SEM and energy dispersive measurements respectively on the sample. The depth of the analysis craters was measured with a Tencor Alpha-Step 300 pro®ler. Electroluminescence (EL) measurements were performed by biasing the diode with a pulse generator at a frequency of 400 Hz and a 10% duty cycle. Low
temperature measurements were performed by mounting the silicon side of the sample to a brass holder in a closed-cycle helium refrigerator. Mounting the sample in this manner ensures good thermal contact and good heat dissipation. The brass holder has a small hole that allows the emission through the silicon substrate to be collected from the back side. Emitted light was analyzed through a 1 m monochromator and detected using a liquid nitrogen cooled Ge detector and standard lock-in techniques.
3. Results and discussion SEM depth pro®le analysis is shown from the quadrupole based SIMS measurement in Fig. 1. This ®gure shows that the Er distribution in the GaN:Er epilayer is uniform. The sample shows a depth structure as measured by SIMS of 1428 nm thick GaN:Er layer on 374 nm of GaN on a Si substrate. The Er levels in the sample were determined by EDX to be 0.85% using standardless quantitation methods. The decay length (84±16% matrix intensity) of the Er signal at the GaN:Er/GaN interface is 38 nm The decay length of the Ga signal at the GaN/Si interface is 25 nm. The Si signal increases from 16 to 84% of the peak intensity at 25 nm. The Si 28 AMU species was followed. The apparent presence of Si within the GaN layer is likely
H. Shen et al. / Solid-State Electronics 43 (1999) 1231±1234
Fig. 2. Room temperature (300 K) electroluminescence from the Er doped GaN diodes after electrical current heating (dashed line) and after thermal annealing (solid line).
to be due to the unresolved detection of the N+ 2 species. Before annealing, the sample exhibits a breakdown
Fig. 3. EL spectra taken from 25±300 K. The current density is held constant at 5 A/cm2.
1233
Fig. 4. Integrated intensity as a function of temperature.
at very high voltages, 050 V. Prior to thermal annealing, though, no emission is evident in either bias direction. However, we noticed that in one diode, after current induced heating (08 W), electroluminescence appeared at 1.54 and 1.35 mm (dashed line in Fig. 2). Subsequently, the samples were annealed in an RTA to form a better alloy to the GaN material. After annealing, all the diodes emitted light when reverse biased beyond 10 V. The devices though exhibited a softer breakdown at 10 V. The solid line of Fig. 2 shows the EL spectra taken for the diode samples under reverse bias conditions after annealing. No emission is observable under forward biased operation. Fig. 3 shows EL spectra taken from 25±300 K with a constant current density of 5 A/cm2. Spectra from the dierent temperatures have similar line-shapes. This broad emission is known to occur in reverse biased junctions [5]. We attribute the 1.54 mm emission to the Er3+ emission. At lower temperatures this feature shifts slightly towards a shorter wavelength (1.53 mm). The origin of the shorter wavelength feature is still undetermined. In Er doped silicon, this feature was attributed to defect bands found in ion-damaged silicon after annealing [6] but in our samples, the silicon substrate is not Er doped; nor is the sample ion implanted, so this is probably not the case. Fig. 4 shows the temperature dependence of the integrated EL intensity. The room temperature intensity is about 12.5% of the 30 K value. The Er doped GaN exhibited bright EL even at room temperature. The
1234
H. Shen et al. / Solid-State Electronics 43 (1999) 1231±1234
4. Conclusions Priolo et al. [8] have studied the temperature dependence of the EL from Er doped silicon. They also observed strong 1.54 mm emission under reverse bias. The quenching of the EL signal, as a function of increasing temperature, is a factor of 8. In general the optical emission of Er3+ ions in semiconductors has not been as ecient as it is in dielectric materials. It is believed that wide band-gap semiconductors serving as a host material should lead to a weaker thermal quenching. This is due to the fact that more ionic bonds are better for forming the required Er3+ energy levels. Our results indicate that GaN EL can exhibit much stronger quenching compared to Er doped silicon. References Fig. 5. Integrated intensity as a function of applied current.
[1] Mears PJ, Reekie L, Jauncey IM, Payne DN. Electron Lett 1987;23:1026.
fact that the emission at room temperature is quenched by 087%, when compared to the 30 K value, indicates that the GaN host material somewhat suppresses the temperature quenching of the Er3+ EL signal. The amount of quenching that we observe is still large compared to previous observations [7] but those samples were explored using below bandgap excitation. During EL, the material is being excited by energies above the bandgap. Fig. 5 shows the dependence of the integrated EL intensity on excitation current. The measurement was performed at room temperature. A threshold for emission was clearly observed, indicating that the mechanism for the Er3+ emission is impact ionization by hot carriers [5].
[2] Desurvire E, Simpson RJ, Becker PC. Optic Lett 1987;12:888. [3] Wilson RG, Schwartz RN, Abernathy CR, Pearton SJ, Newman N, Rubin M, Fu T, Zavada JM. Appl Phys Lett 1994;65:992. [4] Mackenzie JD, Abernathy CR, Pearton SJ, HoÈmmerich U, Wu X, Schwartz RN, Wilson RG, Zavada JM. J Cryst Growth 1997;175/176:84. [5] Polman A. J Appl Phys 1997;82:1. [6] Davies G. Phys Rep 1989;176:83. [7] Thaik M, HoÈmmerich U, Schwartz RN, Wilson RG, Zavada JM. Appl Phys Lett 1997;71:2641. [8] Priolo F, Coa S, Franzo G, Polman A. Mat Res Soc Symp Proc 1996;422:305.