- Email: [email protected]
Characterization of GaP pyramidal textured p–n junctions
PERGAMON
Solid-State Electronics 43 (1999) 779±783
Characterization of GaP pyramidal textured p±n junctions X. Mei a, H.E. Ruda a, *, T. Berdinskihk a, M. Buchanan b a
Electronic Material Group, Department of Metallurgy and Materials Sciences, University of Toronto, 184 College Street, Toronto, Canada M5S 1A4 b National Research Council of Canada, IMS, Montreal Road, Ottawa, Canada K1A 0R6 Received 20 August 1998; accepted 6 October 1998
Abstract GaP pyramidal textured p±n junction (PTJ) diodes prepared using liquid phase epitaxy growth were characterized using ®eld-emission scanning electron microscopy (FE-SEM), electron beam induced current (EBIC), I±V and C±V measurements. FE-SEM images of cross-sectional surfaces of the PTJ diode showed sharp textured p±n junction interfaces and ¯at surfaces. EBIC analysis proved that the current collection follows the textured junction pro®les. I±V characteristics of GaP PTJ and ¯at junction (FJ) diodes prepared under the same conditions both demonstrated very low leakage current, indicating that the formation of PTJ does not introduce extra structure-related defects. Analysis of C±V measurement indicated a 25% increase in junction interface area for the PTJ compared with the ¯at junction devices, in agreement with the estimated area increase of the textured surfaces. The a particle irradiation response of a PTJ device showed an 88% increase in short circuit current as compared with that of a FJ counterpart. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction Typical epitaxially grown junction devices (i.e., p±n junction or hetero-junction) have planar junction interfaces in order to achieve high junction quality. However, non-planar junctions oer an extra degree of freedom for device design, which can be exploited to develop new types of device. For example, GaAs± AlGaAs and (InGa)(AsP)±InP hetero-junctions grown on preferentially etched channels or ribs have been reported for fabricating laterally con®ned or buried lasers [1±4]. (GaIn)(AsP)±InP hetero-junctions with corrugated interfaces were used to fabricate distributed feed back (DFB) lasers [5±7]. Textured p±n junctions can also be used to improve the eciencies of photo or radio voltaic conversion or detection devices, especially for those materials whose minority carrier diusion lengths are much smaller
* Corresponding author. Tel.: +1-416-978-4556; fax: +1416-978-4155 E-mail address: [email protected] (H.E. Ruda)
than the electron±hole (e±h) pair generation range. The use of GaP for a radio voltaic cells or a particle detectors is one such example. The e±h pair generation range of a particles with an energy of 05 MeV (i.e., typical of an Americium source) is almost ten times the sum of minority diusion lengths of electrons and holes in GaP. The use of a textured p±n junction structure is therefore one ecient way to overcome this problem. However, studies on such kind of approach have not been reported. GaP is an important semiconductor material for light emitting and high temperature devices. It can also be used for radio voltaic cells [8, 9] (i.e., b or a cells), and radiation detectors thanks to its advantageous properties of good radiation hardness and very low junction leakage current. We successfully realized GaP pyramidal textured p±n junctions (PTJs) with ¯at surfaces using liquid phase epitaxy (LPE) on pyramidal textured substrates. We believe this device structure is very useful to design high eciency GaP radio voltaic cells, radiation detectors and special light emitting devices.
0038-1101/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 8 - 1 1 0 1 ( 9 8 ) 0 0 3 0 3 - 7
780
X. Mei et al. / Solid-State Electronics 43 (1999) 779±783
In this paper we report on the characterization of GaP PTJs. We present the p±n junction pro®les that were studied using ®eld emission scanning electron microscopy (FE-SEM) and electron beam induced current (EBIC), and the electrical properties of this junctions using I±V and C±V measurements. 2. Experiments The GaP PTJs were prepared using LPE growth of Mg doped layers on n-type pyramidal textured GaP substrates at a growth temperature of 9508C for a growth time of 6 min. The detailed discussion on the growth behavior of LPE layers on pyramidal textured surfaces was reported elsewhere [10]. The pyramidal textured surface pro®le before growth is shown in Fig. 1; this was measured on a pyramidal substrate that was exposed to the heat environment of a growth run, including 2 h baking at 9658C, which was introduced to reduce background doping. The high temperature baking process caused obvious surface erosion as evident from Fig. 1, due to escape of volatile P. The pyramidal substrates were prepared using photolithographic masking of the GaP (111)B surface, and subsequent etching in a C2H4O2:HCl:H2O2=1:1:1 solution for 6 min. Detailed discussions relating to the pyramidal surface preparation were reported previously [11]. In order to compare the properties of GaP PTJs and ¯at junctions (FJs), GaP FJs were also prepared using LPE growth on well polished ¯at GaP (111)B substrates. GaP FJs were grown at the same time and using the same solution as for the PTJs so as to keep identical growth conditions. The substrates for PTJs and FJs were from the same (111) GaP wafer having a
Fig. 1. Field-emission scanning electron microscope image of the surface morphology of GaP pyramidal textured surface before liquid epitaxy, corresponding to 6 min etching in the solution of C2H4O2:HCl:H2O2=1:1:1.
sulfur doping concentration of 05 1017 cmÿ3. The doping concentration in the p-type layer was kept at 03 1017 cmÿ3, as estimated from Hall measurements on GaP layers grown under the same growth conditions on semi-insulating GaP substrates. Ohmic contacts for p- and n-type GaP were achieved using Au±Zn and Au±Ge alloys, respectively. Cross-sectional surface morphology and EBIC analysis were carried out using a Hitachi S-4500 FESEM system. I±V and C±V measurements were conducted on a computer controlled HP 4140B pA meter/ d.c. voltage source and HP 4275 multi-frequency LCR meter, respectively. 3. Characterizations of GaP PTJs 3.1. Junction pro®le and EBIC analysis The secondary electron mode FE-SEM image of a typical PTJ pro®le is shown in Fig. 2(a), which also
Fig. 2. Field-emission scanning electron microscope, (a) secondary electron and (b) electron beam induced current mode image from the cross-sectional surface of the pyramidal textured p±n junction device.
X. Mei et al. / Solid-State Electronics 43 (1999) 779±783
shows the EBIC signal of the p±n junction (i.e., Fig. 2(b)). This reveals a sharp p±n junction interface which follows the pyramidal textured features. The peak position of the EBIC signal is coincident with the p±n junction interface. This peak position follows the textured junction interface while the scanning line moves, as evident from Fig. 2(b). Current collection thus originates from the bright region that is de®ned by the textured p±n junction interface. The secondary electron mode FE-SEM micrograph of the FJ structure is shown in Fig. 3(a). The FJ structure clearly has a ¯at p±n junction interface and a ¯at current collection region, as seen in Fig. 3(a) and (b). From Figs. 2 and 3, the maximum thickness of the layer on the textured substrate, counted from the base ¯oor of the pyramidal surface to the LPE layer surface, is about the same as that on the ¯at substrate. With a textured junction interface, the PTJ device obviously has a larger junction interface area per unit surface area compared to the FJ counterpart. The increase in the junction area can be made much more
Fig. 3. Field-emission scanning electron microscope, (a) secondary electron and (b) electron beam induced current mode image from the cross-sectional surface of the ¯at p±n junction device.
781
signi®cant through judicious choice of the geometric dimensions of the textured features. The EBIC signal width (i.e., the width at 1/e intensity position) and current collection region width for both of the PTJ and FJ devices have about the same value of 1.0 mm as seen from Figs. 2 and 3, indicating similar p±n junction quality for both kinds of devices. This also con®rms that the increase of current collection area directly results in the increase of current collection volume. A small EBIC signal beside the p±n junction is visible for both PTJ and FJ samples in the n-type region. This signal is most probably due to doping variation in this region, or to an n-n + barrier introduced by inter-diusion during LPE growth. 3.2. I±V and C±V characteristics The room temperature I±V characteristics of the PTJ and FJ devices are shown in Fig. 4. The reverse bias leakage current is quite low for both PTJ and FJ devices as seen from Fig. 4. The reverse saturation current, J0, extrapolated from the forward bias curves, has almost the same value of 2 10ÿ15 A cmÿ2 for both the PTJ and FJ devices. This J0 value is among the lowest reported values for a GaP p±n junction diode [12], indicating excellent quality for both the GaP PTJ and FJ devices. The almost identical J0 value for the GaP PTJ and FJ devices further con®rmed that the PTJ device does not have extra structure-related defects. The virtually saturated current region at small forward bias for both PTJ and FJ devices as seen in Fig. 4 can be attributed to leakage from the edges of the samples. This leakage current is probably also the major leakage current source in the reverse bias region since it is at a similar level to the reverse bias leakage
Fig. 4. I±V characteristics of the GaP pyramidal textured (solid line) and ¯at (dashed line) junction devices.
782
X. Mei et al. / Solid-State Electronics 43 (1999) 779±783
Fig. 5. 1/C 2±V curves of the GaP pyramidal textured (solid line) and ¯at (dashed line) junction devices.
Fig. 6. I±V plots of the pyramidal textured (solid line) and ¯at (dashed line) junction devices under a particle irradiation.
current. Introducing a mesa structure or using surface passivation can reduce this leakage current [13]. The 1/C 2±V curves for the GaP PTJ and FJ devices are shown in Fig. 5, where the values of capacitance were normalized to unit area for convenience of comparison. The slightly curved lines of 1/C 2±V functions indicate slight doping variations over thickness of the layers. From Fig. 5, one can see that the capacitance per unit area of the PTJ is obviously larger than that of the FJ. Assuming that the doping concentration is similar in both p- and n-type regions for the PTJ and the FJ devices, respectively, one can estimate the real junction interface area for the PTJ device by comparing the capacitance of the PTJ and FJ devices. Let A 0 be the per unit area of surface (i.e., also called super®cial area ratio) of the PTJ device. A 0 can be estimated using the simple relation, A 0 = CPTJ/CFJ, where CPTJ and CFJ are the capacitance per unit surface area of PTJ and FJ, respectively. According to the results in Fig. 5, A 0 is 1.25, which is in the range of the roughly estimated value according to the geometric dimensions of the pyramidal textured features as shown in Fig. 1. Precise calculation of the area ratio from the geometric shape and dimensions is dicult due to the curved surfaces of the pyramids.
values were also normalized to unit surface area for both PTJ and FJ devices. The a particle generated current (i.e., the short circuit current) for the PTJ device is signi®cantly higher (around 88%) than that for the FJ counterpart. Some improvement in the open circuit voltage for the PTJ device was also recorded, as seen from Fig. 6. The signi®cant improvement in the short circuit current of the PTJ device compared to that of the FJ device con®rmed that the PTJ structure oers a very promising mean for improving the current collection eciency of radio voltaic devices. The increase of the short circuit current of the PTJ device compared to that of the FJ device is much higher than the junction area increase. The extra contribution to the improvement of the short circuit current is believed to be mainly attributed to inhomogeneous distribution of a particle generated e±h pairs with a particle penetration depth. The p±n junction of the FJ is at about the deepest position of that of the PTJ, where the generated e±h pair concentration is relatively lower than that at shallower positions [14]. Therefore, the PTJ device has a relatively large portion of the p±n junction located at the high e±h pair concentration region.
3.3. a particle irradiation response The PTJ and the FJ devices were tested under a particle irradiation emitted from a 241 Am source with a dose of 1 mCi. The a particle energy is 04.6 eV and the power ¯ux is 020 mW cmÿ2. The source to sample spacing is 1 mm. In order to eliminate in¯uences from room light, the tests were carried out in a dark enclosure. The a particle irradiation responses of the PTJ and the FJ devices are shown in Fig. 6. The current
4. Conclusions Characterization of GaP PTJ devices was studied and compared with their FJ counterparts, both prepared under the same LPE growth conditions. FESEM studies show sharp pyramidal textured p±n junction interfaces for the PTJ devices, and ¯at junction interfaces for the FJ samples. EBIC analysis con®rmed that the current collection region of the PTJ device follows the textured junction interface, and has a con-
X. Mei et al. / Solid-State Electronics 43 (1999) 779±783
siderably larger collection area compared to that of the FJ device. I±V characteristics of the GaP PTJ and FJ devices both show very low leakage current. The reverse saturation current J0 for the PTJ and the FJ devices are almost the same with a value of 2 10ÿ15 A cmÿ2, which is among the best-reported results for GaP diodes. This indicates that the formation of the PTJ structure does not introduce extra structure-related defects. A 25% increase in junction interface area of PTJ compared to FJ was deduced from analysis of to C±V results. a particle irradiation responses of the PTJ and FJ devices revealed an 88% increase in the short circuit current of the PTJ compared to FJ counterpart, which con®rmed the eectiveness of increasing the current collection eciency for radio voltaic devices by using PTJ structures. Acknowledgements The authors would like to express their appreciation to Prof. B. Yacobi and Dr. Y. Masaki for their valuable discussions, and to F. Neub and S. Boccia for their technical assistance in making FE-SEM and EBIC measurements.
783
References [1] Kirby PA, Thompson GHB. J Appl Phys 1976;47:4578. [2] Turley SEH, Greene PD. J Cryst Growth 1982;58:409. [3] Botez D, Tsang WT, Wang S. Appl Phys Lett 1976;28:234. [4] Logan RA, Temkin H, Merritt FR, Mahajan S. Appl Phys Lett 1984;45:1275. [5] Doi A, Fukuzama T, Nakamura M, Ito R, AiKi K. Appl Phys Lett 1979;35:441. [6] Nelson AW, Westbrook LD, Evans JS. Electr Lett 1983;19:34. [7] Burkland H, Kuphal E, Dingels HW. Electr Lett 1986;22:802. [8] Walko RJ, Lincoln RC, Baca WE, Goods SH. Proc 26th Int Energy Conversion Eng Conf 1991;6:135. [9] Huges RC, Ziperian TE, Dawson LR, Biefeld RM. Bull Am Phys Soc 1989;34:999. [10] Mei X, Ruda HE, Berdiskihk T, Buchanan M. J Cryst Growth 1998;193:148. [11] Berdinskihk T, Ruda HE, Mei X, Buchanan M. J Electron Mat 1998;27:114. [12] Hughes RC, Zipperian TE, Dawson LR, Biefeld RM, Walko RJ, Dvorak MA. J Appl Phys 1991;69:6500. [13] Jedral L, Ruda HE, Sodhi R, Ma H, Mannik L. Can J Phys 1992;70:1050. [14] Colerico CW, Serreze HB, Messenger SR, Xapsos MA, Bueke EA. IEEE Trans Nucl Sci 1995;42:2089.
Solid-State Electronics 43 (1999) 779±783
Characterization of GaP pyramidal textured p±n junctions X. Mei a, H.E. Ruda a, *, T. Berdinskihk a, M. Buchanan b a
Electronic Material Group, Department of Metallurgy and Materials Sciences, University of Toronto, 184 College Street, Toronto, Canada M5S 1A4 b National Research Council of Canada, IMS, Montreal Road, Ottawa, Canada K1A 0R6 Received 20 August 1998; accepted 6 October 1998
Abstract GaP pyramidal textured p±n junction (PTJ) diodes prepared using liquid phase epitaxy growth were characterized using ®eld-emission scanning electron microscopy (FE-SEM), electron beam induced current (EBIC), I±V and C±V measurements. FE-SEM images of cross-sectional surfaces of the PTJ diode showed sharp textured p±n junction interfaces and ¯at surfaces. EBIC analysis proved that the current collection follows the textured junction pro®les. I±V characteristics of GaP PTJ and ¯at junction (FJ) diodes prepared under the same conditions both demonstrated very low leakage current, indicating that the formation of PTJ does not introduce extra structure-related defects. Analysis of C±V measurement indicated a 25% increase in junction interface area for the PTJ compared with the ¯at junction devices, in agreement with the estimated area increase of the textured surfaces. The a particle irradiation response of a PTJ device showed an 88% increase in short circuit current as compared with that of a FJ counterpart. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction Typical epitaxially grown junction devices (i.e., p±n junction or hetero-junction) have planar junction interfaces in order to achieve high junction quality. However, non-planar junctions oer an extra degree of freedom for device design, which can be exploited to develop new types of device. For example, GaAs± AlGaAs and (InGa)(AsP)±InP hetero-junctions grown on preferentially etched channels or ribs have been reported for fabricating laterally con®ned or buried lasers [1±4]. (GaIn)(AsP)±InP hetero-junctions with corrugated interfaces were used to fabricate distributed feed back (DFB) lasers [5±7]. Textured p±n junctions can also be used to improve the eciencies of photo or radio voltaic conversion or detection devices, especially for those materials whose minority carrier diusion lengths are much smaller
* Corresponding author. Tel.: +1-416-978-4556; fax: +1416-978-4155 E-mail address: [email protected] (H.E. Ruda)
than the electron±hole (e±h) pair generation range. The use of GaP for a radio voltaic cells or a particle detectors is one such example. The e±h pair generation range of a particles with an energy of 05 MeV (i.e., typical of an Americium source) is almost ten times the sum of minority diusion lengths of electrons and holes in GaP. The use of a textured p±n junction structure is therefore one ecient way to overcome this problem. However, studies on such kind of approach have not been reported. GaP is an important semiconductor material for light emitting and high temperature devices. It can also be used for radio voltaic cells [8, 9] (i.e., b or a cells), and radiation detectors thanks to its advantageous properties of good radiation hardness and very low junction leakage current. We successfully realized GaP pyramidal textured p±n junctions (PTJs) with ¯at surfaces using liquid phase epitaxy (LPE) on pyramidal textured substrates. We believe this device structure is very useful to design high eciency GaP radio voltaic cells, radiation detectors and special light emitting devices.
0038-1101/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 8 - 1 1 0 1 ( 9 8 ) 0 0 3 0 3 - 7
780
X. Mei et al. / Solid-State Electronics 43 (1999) 779±783
In this paper we report on the characterization of GaP PTJs. We present the p±n junction pro®les that were studied using ®eld emission scanning electron microscopy (FE-SEM) and electron beam induced current (EBIC), and the electrical properties of this junctions using I±V and C±V measurements. 2. Experiments The GaP PTJs were prepared using LPE growth of Mg doped layers on n-type pyramidal textured GaP substrates at a growth temperature of 9508C for a growth time of 6 min. The detailed discussion on the growth behavior of LPE layers on pyramidal textured surfaces was reported elsewhere [10]. The pyramidal textured surface pro®le before growth is shown in Fig. 1; this was measured on a pyramidal substrate that was exposed to the heat environment of a growth run, including 2 h baking at 9658C, which was introduced to reduce background doping. The high temperature baking process caused obvious surface erosion as evident from Fig. 1, due to escape of volatile P. The pyramidal substrates were prepared using photolithographic masking of the GaP (111)B surface, and subsequent etching in a C2H4O2:HCl:H2O2=1:1:1 solution for 6 min. Detailed discussions relating to the pyramidal surface preparation were reported previously [11]. In order to compare the properties of GaP PTJs and ¯at junctions (FJs), GaP FJs were also prepared using LPE growth on well polished ¯at GaP (111)B substrates. GaP FJs were grown at the same time and using the same solution as for the PTJs so as to keep identical growth conditions. The substrates for PTJs and FJs were from the same (111) GaP wafer having a
Fig. 1. Field-emission scanning electron microscope image of the surface morphology of GaP pyramidal textured surface before liquid epitaxy, corresponding to 6 min etching in the solution of C2H4O2:HCl:H2O2=1:1:1.
sulfur doping concentration of 05 1017 cmÿ3. The doping concentration in the p-type layer was kept at 03 1017 cmÿ3, as estimated from Hall measurements on GaP layers grown under the same growth conditions on semi-insulating GaP substrates. Ohmic contacts for p- and n-type GaP were achieved using Au±Zn and Au±Ge alloys, respectively. Cross-sectional surface morphology and EBIC analysis were carried out using a Hitachi S-4500 FESEM system. I±V and C±V measurements were conducted on a computer controlled HP 4140B pA meter/ d.c. voltage source and HP 4275 multi-frequency LCR meter, respectively. 3. Characterizations of GaP PTJs 3.1. Junction pro®le and EBIC analysis The secondary electron mode FE-SEM image of a typical PTJ pro®le is shown in Fig. 2(a), which also
Fig. 2. Field-emission scanning electron microscope, (a) secondary electron and (b) electron beam induced current mode image from the cross-sectional surface of the pyramidal textured p±n junction device.
X. Mei et al. / Solid-State Electronics 43 (1999) 779±783
shows the EBIC signal of the p±n junction (i.e., Fig. 2(b)). This reveals a sharp p±n junction interface which follows the pyramidal textured features. The peak position of the EBIC signal is coincident with the p±n junction interface. This peak position follows the textured junction interface while the scanning line moves, as evident from Fig. 2(b). Current collection thus originates from the bright region that is de®ned by the textured p±n junction interface. The secondary electron mode FE-SEM micrograph of the FJ structure is shown in Fig. 3(a). The FJ structure clearly has a ¯at p±n junction interface and a ¯at current collection region, as seen in Fig. 3(a) and (b). From Figs. 2 and 3, the maximum thickness of the layer on the textured substrate, counted from the base ¯oor of the pyramidal surface to the LPE layer surface, is about the same as that on the ¯at substrate. With a textured junction interface, the PTJ device obviously has a larger junction interface area per unit surface area compared to the FJ counterpart. The increase in the junction area can be made much more
Fig. 3. Field-emission scanning electron microscope, (a) secondary electron and (b) electron beam induced current mode image from the cross-sectional surface of the ¯at p±n junction device.
781
signi®cant through judicious choice of the geometric dimensions of the textured features. The EBIC signal width (i.e., the width at 1/e intensity position) and current collection region width for both of the PTJ and FJ devices have about the same value of 1.0 mm as seen from Figs. 2 and 3, indicating similar p±n junction quality for both kinds of devices. This also con®rms that the increase of current collection area directly results in the increase of current collection volume. A small EBIC signal beside the p±n junction is visible for both PTJ and FJ samples in the n-type region. This signal is most probably due to doping variation in this region, or to an n-n + barrier introduced by inter-diusion during LPE growth. 3.2. I±V and C±V characteristics The room temperature I±V characteristics of the PTJ and FJ devices are shown in Fig. 4. The reverse bias leakage current is quite low for both PTJ and FJ devices as seen from Fig. 4. The reverse saturation current, J0, extrapolated from the forward bias curves, has almost the same value of 2 10ÿ15 A cmÿ2 for both the PTJ and FJ devices. This J0 value is among the lowest reported values for a GaP p±n junction diode [12], indicating excellent quality for both the GaP PTJ and FJ devices. The almost identical J0 value for the GaP PTJ and FJ devices further con®rmed that the PTJ device does not have extra structure-related defects. The virtually saturated current region at small forward bias for both PTJ and FJ devices as seen in Fig. 4 can be attributed to leakage from the edges of the samples. This leakage current is probably also the major leakage current source in the reverse bias region since it is at a similar level to the reverse bias leakage
Fig. 4. I±V characteristics of the GaP pyramidal textured (solid line) and ¯at (dashed line) junction devices.
782
X. Mei et al. / Solid-State Electronics 43 (1999) 779±783
Fig. 5. 1/C 2±V curves of the GaP pyramidal textured (solid line) and ¯at (dashed line) junction devices.
Fig. 6. I±V plots of the pyramidal textured (solid line) and ¯at (dashed line) junction devices under a particle irradiation.
current. Introducing a mesa structure or using surface passivation can reduce this leakage current [13]. The 1/C 2±V curves for the GaP PTJ and FJ devices are shown in Fig. 5, where the values of capacitance were normalized to unit area for convenience of comparison. The slightly curved lines of 1/C 2±V functions indicate slight doping variations over thickness of the layers. From Fig. 5, one can see that the capacitance per unit area of the PTJ is obviously larger than that of the FJ. Assuming that the doping concentration is similar in both p- and n-type regions for the PTJ and the FJ devices, respectively, one can estimate the real junction interface area for the PTJ device by comparing the capacitance of the PTJ and FJ devices. Let A 0 be the per unit area of surface (i.e., also called super®cial area ratio) of the PTJ device. A 0 can be estimated using the simple relation, A 0 = CPTJ/CFJ, where CPTJ and CFJ are the capacitance per unit surface area of PTJ and FJ, respectively. According to the results in Fig. 5, A 0 is 1.25, which is in the range of the roughly estimated value according to the geometric dimensions of the pyramidal textured features as shown in Fig. 1. Precise calculation of the area ratio from the geometric shape and dimensions is dicult due to the curved surfaces of the pyramids.
values were also normalized to unit surface area for both PTJ and FJ devices. The a particle generated current (i.e., the short circuit current) for the PTJ device is signi®cantly higher (around 88%) than that for the FJ counterpart. Some improvement in the open circuit voltage for the PTJ device was also recorded, as seen from Fig. 6. The signi®cant improvement in the short circuit current of the PTJ device compared to that of the FJ device con®rmed that the PTJ structure oers a very promising mean for improving the current collection eciency of radio voltaic devices. The increase of the short circuit current of the PTJ device compared to that of the FJ device is much higher than the junction area increase. The extra contribution to the improvement of the short circuit current is believed to be mainly attributed to inhomogeneous distribution of a particle generated e±h pairs with a particle penetration depth. The p±n junction of the FJ is at about the deepest position of that of the PTJ, where the generated e±h pair concentration is relatively lower than that at shallower positions [14]. Therefore, the PTJ device has a relatively large portion of the p±n junction located at the high e±h pair concentration region.
3.3. a particle irradiation response The PTJ and the FJ devices were tested under a particle irradiation emitted from a 241 Am source with a dose of 1 mCi. The a particle energy is 04.6 eV and the power ¯ux is 020 mW cmÿ2. The source to sample spacing is 1 mm. In order to eliminate in¯uences from room light, the tests were carried out in a dark enclosure. The a particle irradiation responses of the PTJ and the FJ devices are shown in Fig. 6. The current
4. Conclusions Characterization of GaP PTJ devices was studied and compared with their FJ counterparts, both prepared under the same LPE growth conditions. FESEM studies show sharp pyramidal textured p±n junction interfaces for the PTJ devices, and ¯at junction interfaces for the FJ samples. EBIC analysis con®rmed that the current collection region of the PTJ device follows the textured junction interface, and has a con-
X. Mei et al. / Solid-State Electronics 43 (1999) 779±783
siderably larger collection area compared to that of the FJ device. I±V characteristics of the GaP PTJ and FJ devices both show very low leakage current. The reverse saturation current J0 for the PTJ and the FJ devices are almost the same with a value of 2 10ÿ15 A cmÿ2, which is among the best-reported results for GaP diodes. This indicates that the formation of the PTJ structure does not introduce extra structure-related defects. A 25% increase in junction interface area of PTJ compared to FJ was deduced from analysis of to C±V results. a particle irradiation responses of the PTJ and FJ devices revealed an 88% increase in the short circuit current of the PTJ compared to FJ counterpart, which con®rmed the eectiveness of increasing the current collection eciency for radio voltaic devices by using PTJ structures. Acknowledgements The authors would like to express their appreciation to Prof. B. Yacobi and Dr. Y. Masaki for their valuable discussions, and to F. Neub and S. Boccia for their technical assistance in making FE-SEM and EBIC measurements.
783
References [1] Kirby PA, Thompson GHB. J Appl Phys 1976;47:4578. [2] Turley SEH, Greene PD. J Cryst Growth 1982;58:409. [3] Botez D, Tsang WT, Wang S. Appl Phys Lett 1976;28:234. [4] Logan RA, Temkin H, Merritt FR, Mahajan S. Appl Phys Lett 1984;45:1275. [5] Doi A, Fukuzama T, Nakamura M, Ito R, AiKi K. Appl Phys Lett 1979;35:441. [6] Nelson AW, Westbrook LD, Evans JS. Electr Lett 1983;19:34. [7] Burkland H, Kuphal E, Dingels HW. Electr Lett 1986;22:802. [8] Walko RJ, Lincoln RC, Baca WE, Goods SH. Proc 26th Int Energy Conversion Eng Conf 1991;6:135. [9] Huges RC, Ziperian TE, Dawson LR, Biefeld RM. Bull Am Phys Soc 1989;34:999. [10] Mei X, Ruda HE, Berdiskihk T, Buchanan M. J Cryst Growth 1998;193:148. [11] Berdinskihk T, Ruda HE, Mei X, Buchanan M. J Electron Mat 1998;27:114. [12] Hughes RC, Zipperian TE, Dawson LR, Biefeld RM, Walko RJ, Dvorak MA. J Appl Phys 1991;69:6500. [13] Jedral L, Ruda HE, Sodhi R, Ma H, Mannik L. Can J Phys 1992;70:1050. [14] Colerico CW, Serreze HB, Messenger SR, Xapsos MA, Bueke EA. IEEE Trans Nucl Sci 1995;42:2089.