Ga 1-x. Study of PV characteristics of Al x. As/GaAs photodiodes. Journal of Physics: Conference Series PAPER OPEN ACCESS

Size: px

Start display at page:

Download "Ga 1-x. Study of PV characteristics of Al x. As/GaAs photodiodes. Journal of Physics: Conference Series PAPER OPEN ACCESS"

Byron Sullivan
5 years ago
Views:

Journa of Physics: Conference Series PAPER OPEN ACCESS Study of PV characteristics of A x Ga -x As/GaAs photodiodes To cite this artice: V S Kainovskiy

1 Journa of Physics: Conference Series PAPER OPEN ACCESS Study of PV characteristics of A x Ga -x As/GaAs photodiodes To cite this artice: V S Kainovskiy et a 08 J. Phys.: Conf. Ser View the artice onine for updates and enhancements. This content was downoaded from IP address on 0/0/08 at 0:46

2 Study of PV characteristics of A x Ga -x As/GaAs photodiodes V S Kainovskiy, E V Kontrosh, G A Gusev, A N Sumarokov, G V Kimko, S V Ivanov, V S Yuferev, T S Tabarov and V M Andreev Ioffe Institute, 6 Poytekhnicheskaya, St. Petersburg, Russian Federation, 940 Peter The Great St. Petersburg Poytechnic University, 9 Poytekhnicheskaya, St. Petersburg, Russian Federation, 955 The First Eectrotechnica University, Pr. Popov 5, St. Petersburg, Russian Federation, 9776 E-mai: vitak.sopt@mai.ioffe.ru Abstract. Simuation of the structure of mutijunction photodiodes has been performed on the assumption that an equa of the number of photons is absorbed in them, and PV characteristics have been stadied A x Ga -x As/GaAs photodiodes fabricated by the moecuar-beam epitaxy technique. According to the performed measurements and anaysis of the resuts, it has been shown that the efficiency of singe-junction photodiodes in converting monochromatic optica radiation at a waveength of 80 nm reaches 50%. The reationship between the saturation currents cacuated from the dark I - V characteristic for the diffusion current fow (Shockey) in the space charge region of a photodiode p-n junction and the obtained vaues for the efficiency of converting optica radiation in the waveength range of nm has been shown. When the saturation current fow for the diffusion mechanism rises by an order of magnitude, the efficiency vaue at excitation by monochromatic radiation at 80 nm and 780 nm drops by 5% and %, respectivey.. Introduction In recent years, appication of mutijunction nano-dimensiona heterostructures of III-V photoactive p-n junctions with forbidden gaps increasing towards the fronta surface and connected by tunne diodes was, mainy, considered for creating efficient sunight converters mutijunction soar ces (MJ SCs) []. At present, MJ nano-dimensiona heterostructures find their appication aso in creating high-efficient photodiodes (PDs) converting monochromatic radiation [, ]. Unike an MJ SC, a monoithic PD consists of severa p-n junctions with simiar forbidden gaps of the approximate composition, but with different geometries of junctions connected by tunne diodes. In the PV mode of PD operation, this aows conversion of monochromatic radiation with higher votage of the output signa and improvement of conditions for correation with the oad. In the present paper, the structure of a monoithic tripe-junction p-i-n A x Ga -x As/GaAs PD has been simuated. Dark and oad I-V characteristics of singe-junction p-i-n A x Ga -x As/GaAs PDs were investigated experimentay under excitation by continuous monochromatic radiation at a waveength of 80 nm at a power of up to 400 mw and at 780 nm at a power of up to. W. Spectra characteristics of the externa quantum yied of a p-i-n A x Ga -x As/GaAs PD with a sma photosensitive surface (500 μm in diameter) were recorded. Content from this work may be used under the terms of the Creative Commons Attribution.0 icence. Any further distribution of this work must maintain attribution to the author(s) and the tite of the work, journa citation and DOI. Pubished under icence by Ltd

3 . Simuation of mutijunction and singe-junction A x Ga -x As/GaAs PD structures A mode was cacuated for a monoithic tripe-junction p-i-n PD based on the A x Ga -x As/GaAs nanoheterostructure. The schematic of the MJ PD is presented in figure. Each junction thickness was determined under the condition of equaity of the number of absorbed radiation photons according to the expressions (-5) presented beow. The tota amount of absorbed radiation in a PD was determined from the expression: I0 = I I out, () where I is the incident radiation intensity (number of photons per сm ), and I out is the intensity of radiation that passes through. The amount of radiation absorbed in the ith junction of an MJ PD with aowing for absorption ony in p, i and n ayers is: I 0i = I ( e i ( α pi pi α ii ii α ni ni ) ), () where I i is the intensity of radiation entering a junction; α is the absorption coefficient of a corresponding ayer (p, i or n), and is the thickness of a corresponding ayer (p, i or n). The amount of radiation absorbed in the first junction at i = with aowance for optica osses in the wideband window is: I =I - I win, () where I win is the amount of radiation absorbed in the wideband window ocated in the first junction structure. Figure. Schematic of a tripe-junction A x Ga -x As/GaAs PD. The amount of radiation absorbed in the beow ocated junctions at < i N with aowance for optica absorption in tunne n ++ -p ++ junctions is: I i = I ( α p,( p,( α i,( i,( α n,( n,( ) i e ITD i, (4)

4 where I TD is the amount of radiation absorbed in a tunne diode. Then, the portion of radiation absorbed in each junction with aowance for photocurrent equaity in a MJ PD junctions is: I 0i I I win i= = N N I TD, i I out. (5) The cacuated thicknesses of p, i, n ayers for each junction and doping eves are presented in figure. Numbers, and denote junctions, and number 4 represents connecting tunne junctions in correspondence with the structura schematic presented in figure. impurity concentration, cm n++/p++ tunne junction x, μm Figure. Schematic of a tripe-junction monoithic p-i-n A x Ga -x As/GaAs PD with indicated cacuated thicknesses of p-i-n junctions and n ++ -p ++ connecting diodes aong the x-axis, and rated doping eves shown aong the y-axis: () top, () midde, () bottom, (4) tunne junctions.. Static and dynamic characteristics of singe-junction p-i-n GaAs/A x Ga -x As PDs The heterostructure of A 0. Ga 0.88 As/GaAs fabricated by moecuar-beam epitaxy was used to investigate characteristics of singe-junction p-i-n PDs with frontay entering monochromatic radiation (figure ). PD chips sized mm mm were formed on the grown epitaxia wafer. Directy on the epitaxia wafer, dark I-V characteristics of singe-junction PDs were recorded within the forward votage bias range of V. The forward dark I-V characteristics were anayzed according to the procedure described in [4, 5], using a three-component exponentia mode of a PD I-V characteristic:

5 J = J i = 0i V (exp( ϕ ) ) Aε Obtained were the vaues of the pre-exponentia factors ( saturation currents for the main mechanisms of minority charge carrier transport), J 0i, diode coefficients (ideaity coefficients), A i, and structure series resistance, R s, for three minority charge carrier transport mechanisms in the space charge region of a p-i-n junction: tunne-trap ( overabundant ) with a diode coefficient А t greater than, recombination (Sah-Noyce-Shockey) with А r = and diffusion (Shockey) with А d =. i (6) window p-a 0. Ga 0.88 As, cm - p-gaas, 0 8 cm - i- region n 0 -GaAs, 0 4 cm - base n-gaas, cm - n- A 0. Ga 0.8 As, 0 8 cm - substrate n + - GaAs, (-) 0 8 cm - Figure. Heterostructure of A x Ga -x As /GaAs p-i-n PDs fabricated by the MBE technique. From the above-mentioned PDs whose characteristics were recorded and anayzed, those with different vaues of pre-exponentia factors corresponding to the diffusion component J 0d were seected. The seection was made for estimating the infuence of the pre-exponentia factors J 0d on the singejunction PD efficiency in the case of converting monochromatic radiation in the range of nm. The dark I-V characteristics of the seected PDs are shown in figure 4 and marked as b, d, and b0, which corresponds to the indices on the epitaxia wafer. Tabe presents the rated vaues of saturation currents J 0t, J 0r, J 0d (pre-exponentia factors) for the tunne-trap, recombination and diffusion current fow mechanisms and, accordingy, the specific series resistance R s of a p-i-n junction structure, a obtained in anayzing experimenta dark I-V characteristics (figure 4). To determine the spectra range of the sensitivity of a tested PD epitaxia structure, externa quantum efficiency characteristics were recorded in the waveength range of nm. The dependences are presented in figure 5. For the seected PDs, oad I-V characteristics were recorded upon exposure to monochromatic W radiation at a waveength of 80 nm. The efficiency vaues cacuated from the oad I-V characteristics in dependence on the incident radiation power ( W) are presented in figure 6 (curves, and ) and tabe. Based on the data presented in figure 6 and tabe, it foows that the b PD with a minima saturation current density vaue J 0d = 0-0 А/сm has the maximum vaue of the efficiency in conversion of constant aser radiation (50%) at a waveength of 80 nm and a power of 0.4 W. The b0 PD with the maximum vaue of J 0d = 0-9 А/сm and R s > 0 mohm сm has the minimum efficiency of 7.8% at an incident aser radiation power of 0.4 W. Figure 6 and tabe present rated vaues of the efficiency for peak vaues of puse ampitude of the photo-response for tested singe-junction A x Ga -x As/GaAs p-i-n PDs. According to the obtained characteristics (figure 6, curves 4, 5 and 6, tabe ), the b PD having the minimum saturation 4

6 current density vaue converts pused radiation at a waveength of 780 nm with the maximum efficiency of 5%. The decrease in the PD efficiency under excitation at a waveength of 780 nm, compared to that at continuous radiation impact at 80 nm, occurs, first of a, by owering the PD spectra sensitivity at 780 nm (figure 5). J,A/cm Figure 4. Dark I-V characteristics for the seected PDs b (curve ), d (curve ), b0 (curve ) with different vaues of pre-exponentia factors for three current fow mechanisms: tunne-trap, J 0t, (А > ); recombination, J 0r, (А = ); diffusion, J 0d, (А = ), and the vaue of the resistance R S. U, V Q ext, % λ, nm Figure 5. Spectra characteristics of the externa quantum efficiency of the structure of an A x Ga -x As/GaAs p-i-n PD with a photoactive surface having a diameter of 500 µm. 5

7 Tabe. No of PD Efficiency, % (λ = 85 nm) R s, (mоhm сm ) J 0t, (0-8 A/сm ) (А > ) J 0r, (0-0 А/сm ) A = ) Р опт = 0. (W) 0.8 (W) 0.4 (W) b d b > J 0d, (0-0 А/сm ) (А = ) η const, % η puse,% P opt, W 0 Figure 6. Dependence of the efficiency on incident power for PDs b (curves,4), d (curves,5) and b0 (curves,6) in a PV mode at excitation by monochromatic radiation: curves,,, at continuous excitation at a waveength of 80 nm (5 0 С); curves 4, 5, 6, at excitation at a waveength of 780 nm. Tabe. No of PD Efficiency, % (λ = 780 nm) P opt = 0.5 (W).0 (W). (W) b d b The parameters cacuated from dark and oad I-V characteristics and fast operation characteristics of singe-junction PDs were used to buid the dependences of the efficiency on saturation current corresponding to the diffusion mechanism for the minority charge carrier transport in the continuous generation mode (figure 7, curve ) and in the pused one (figure 7, curve ). 6

8 η,% J 0d, A/cm Figure 7. Dependences of the PD efficiency on saturation current for the diffusion mechanism, obtained from dark current-votage characteristics of photodiode sampes (b, d, b0) in the continuous generation mode (curve ) and in the pused one (curve ). From the dependences in figures 6 and 7, the reationship between the saturation currents for the diffusion mechanism of minority charge carrier fow in the space charge region of photoactive p-i-n junctions and the efficiency of optica radiation conversion is we pronounced. The smaer the vaues of pre-exponentia factors (J 0d for the diffusion current fow mechanism), the smaer the PD efficiency. The growth of J 0d by an order of magnitude decreases the efficiency by more than 0% in the aser excitation mode at a waveength of 88 nm and by 4% for pused excitation at 780 nm. 4. Concusion A rated mode of a tripe-junction A x Ga -x As/GaAs photodiode for converting powerfu monochromatic radiation in the waveength range of nm is proposed. Investigation of static and dynamic characteristics of singe-junction p-i-n A x Ga -x As/GaAs PDs has been carried out. According to this investigation, it has been shown that the greater the saturation current vaues J 0d corresponding to the diffusion current fow, the smaer the PD efficiency vaue at excitation by monochromatic radiation. For the tested p-i-n PDs, the growth of the saturation currents J 0d by an order of magnitude causes the efficiency to decrease by more than 0% in the mode of excitation by continuous monochromatic radiation at a waveength of 80 nm and by 4% in the modes of excitation by monochromatic puse radiation at 780 nm. Acknowedgements The work has been supported by the Russian Science Foundation (Agreement No ). References [] Aferov Zh I, Andreev V M and Rumyantsev V D Soar photovotaics: Trends and prospects 004 Semiconductors 8,

9 [] Fafard S, York M C A, Proux F, Vadivia C E, Wikins M M, Arès R, Aimez V, Hinzer K and Masson D P Utrahigh efficiencies in vertica epitaxia heterostructure architectures 06 Appied Physics Letters [] Schubert J, Oiva E, Dimroth F, Guter W, Loeckenhoff R and Bett A. W. High-Votage GaAs Photovotaic Laser Power Converters 009 IEEE Transactions on Eectron Device [4] Andreev V M, Evstropov V V, Kainovskiy V S, Lantratov V M and Khvostikov V P 009 Current fow and potentia efficiency of soar ces based on GaAs and GaSb p-n junctions Semiconductors [5] Andreev V M et a 00 Current fow and efficiency of Ge p-n junctions in tripe-junction GaInP/Ga(In)As/Ge soar ces for space appications Proc. 5th European Photovotaic Soar Energy Conference and 5 th Word Conference on Photovotaic Energy Conversion (Vaencia) 979 8

Solar Cell Parameters and Equivalent Circuit

9 Solar Cell Parameters and Equivalent Circuit 9.1 External solar cell parameters The main parameters that are used to characterise the performance of solar cells are the peak power P max, the short-circuit