High open-circuit voltage betavoltaic cellbased on GaN pin homojunction

Z.J. Cheng, H.S. San, Z.H. Feng, B. Liu and X.Y. Chen

A high open-circuit voltage betavoltaic cell based on a GaN pin homo-junction is demonstrated. A process of doping compensation has beendeveloped to achieve high resistance i-GaN film for betavoltaics. Under0.5 mCi 63Ni source irradiation, the open-circuit voltage of the fabri-cated GaN pin homojunction betavoltaic cell was measured as highas 1.65 V. A fill factor of 54% and a 2.7% lower bound on thepower conversion efficiency were obtained. The results suggest GaNis a highly potential candidate for the long-life betavoltaic cell.

Introduction: Semiconductor pn junctions for the conversion of betaradiation into electrical power were first suggested in the 1950s [1]. Inthe latest decade, several betavoltaic cells using a Si pn junctiondevice and a SiC pn, pin or Schottky barrier diode have been fabricated[2–5]. Theoretical analysis shows that the efficiency of betavoltaic con-version increases with increase of the bandgap of the semiconductor [6].Compared with Si (bandgap of 1.12 eV) and SiC (bandgap of 2.3–3.3 eV), the wide bandgap semiconductor GaN (3.4 eV) is a more attrac-tive choice for creating the betavoltaic cell. In addition, GaN has theadvantage of higher radiation resistance. However, no fabricated GaNbetavoltaic cell has been reported so far. Among the parameters thatcontrol the performance of GaN betavoltaic cells, the doping concen-tration of the GaN layer is one of the most influential factors. Lightdoping can achieve a highly resistive (HR) GaN layer with which thedepletion region is widened greatly, thereby increasing the collectionrate of generated electron-hole pairs (EHPs) and enhancing the celloutputs. At present, metal-organic chemical-vapour deposition(MOCVD) is the leading method to grow a GaN. The difficulty relatingto MOCVD GaN is to grow a high quality, thick enough HR GaN layer.Nominally undoped GaN grown on sapphire substrates typically showsn-type conductivity (n . 5 × 1016 cm23) due to residual impurities.Besides, the hole concentration of MOCVD P-GaN is in the limitedrange of 1–5 × 1017 cm23 [7, 8]. These limitations result in toonarrow width of the depletion region of the GaN pn junction structureand decrease of the collection rate of generated EHPs.

In this Letter, we present a GaN pin homojunction betavoltaic cell.The HR i-GaN layer is achieved by the process of doping compensationin which an acceptor impurity of Fe is added to the unintentionallydoped GaN (u-GaN) layer. A GaN pun homojunction betavoltaic cellwas also fabricated and tested as a comparison. 63Ni is used as thebeta radiant source owing to the long half life (100 years) and lowlevel beta radiation for safety considerations (the maximum betaenergy is 66.9 keV and average beta energy is 17.4 keV for 63Ni). I-Vmeasurements were performed. The GaN pin homojunction betavoltaiccell exhibits a high open-circuit voltage (Voc) of 1.65 V under 0.5-mCi63Ni source irradiation.

1000 nm n-GaN n=3×1018 cm–3

GaN buffer layer

u-GaN

600 nm i-GaN n=1×1015 cm–3

sapphire substrate

Ti/Al/Ti/Au

50 nm p-GaN p=2×1017 cm–3

Ni/Au

Ni/Ag/Au

Fig. 1 Schematic structure of GaN pin betavoltaic cell

Experiment: The GaN layers were grown on 2-inch c-plane sapphire(Al2O3) substrates using a Thomas Swan MOCVD reactor system.The pin structure consists of a 1000 nm Si-doped n-GaN layer, a600 nm Fe-doped i-GaN layer, and a 50 nm Mg-doped p-GaN layer.

ELECTRONICS LETTERS 9th June 2011 Vol. 47

According to the same growth condition, the approximated free-electronconcentration in the Fe-doped i-GaN layer is 1 × 1015 cm23 based onprevious growth calibrations. Fig. 1 shows the typical device structurewith details of epitaxial layers. A pun structure consisting of a1000 nm Si-doped n-GaN layer, a 600 nm unintentionally dopedu-GaN layer, and a 50 nm Mg-doped p-GaN layer was also grown asreference for comparison.

For fabrication of GaN betavoltaic cells, first, the p-type GaN layerswere patterned and etched using an inductively coupled plasma (ICP)etching system to expose the n-type GaN layers for preparing theohmic contacts. Then an electron-beam evaporator was used todeposit the contacts consisting of Ti/Al/Ti/Au (26/230/8/80 nm)shown in Fig. 1, annealed at 550 8C for 45 s in an N2 ambient. Next,the Ni/Au (8/10 nm) layers were deposited to form current spreadinglayers on the top p-type GaN layers, annealed at 470 8C for 120s in anitrogen-oxygen mixture (N2: O2 ¼ 2:1) ambient. Finally, the pads con-sisting of Ni/Ag/Au (100/100/200 nm) were deposited on thep-current spreading layers for external contacts. The size of the fabri-cated single cell is 2 × 2 mm2.

The GaN betavoltaic cells were characterised using the KeithleyModel 4200-SCS Semiconductor Characterisation System. Themeasurements were made under dark and room temperature conditionsin a Faraday cage. The 63Ni beta source used in this experiment was a4 × 4 mm2 solid plate with the activity of 2 mCi, supplied by theChina Institute of Atomic Energy.

Results: Fig. 2 shows the dark I-V characteristics of the two types ofGaN betavoltaic cells measured without 63Ni beta irradiation, thecurrent on reverse bias voltage is shown in a logarithm scale andlinear scale on forward. It can be seen that the pin structure has abetter pn junction performance. The leakage current at 25 V is3.95 × 1029 A for the pun cell and 1.68 × 10210 A for the pin cell.The lower leakage current of the pin structure can indicate lowerdefect levels and dislocations compared with the unintentionallydoped pun structure [9]. It is interesting to observe that in the reversedirection both cells exhibit a negative differential resistance between 0and 22 V, the mechanism needed for further research.

–4 –2 0 2 4 6

0.02

0.04

0.06

0.08

0

0.10

voltage, A

curr

ent,

A

curr

ent,

A

1¥10–10

1¥10–8

1¥10–6

1¥10–4

1¥10–12

1¥10–2

punpin

Fig. 2 Dark I-V characteristics of GaN pin and pun homojunction cellsbefore irradiation

0–600

–400

–200

0

200

400

600

800

0.4 0.8 1.2 1.6 2.0

voltage, V

curr

ent,

pA

pun darkpun irradiationpin darkpin irradiation

Fig. 3 I-V characteristics of GaN pin and pun homojunction betavoltaic cellsbefore and after 63Ni source irradiation

Fig. 3 shows the I-V curves of the two cells before and after coveringwith a 4 × 4 mm2 planar solid 63Ni source with an activity of 2 mCi. Asshown in Fig. 3, both cells exhibit marked betavoltaic effects under theirradiation of 63Ni source. For the pun homojunction device, the open-circuit voltage (Voc) and short-circuit current (Isc) are 0.54 V and330 pA, respectively. Meanwhile, the performance of the pin homojunc-tion betavoltaic cell with the HR i-GaN layer due to doping compen-sation is improved significantly. The pin homojunction device has a

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very high open-circuit voltage (Voc) of 1.65 V and a short-circuit current(Isc) of 480 pA. Both devices have similar fill factor (FF) of 54% (for thepin structure) and 56% (for the pun structure). It is worth noting that theVoc of 1.65 V is the record value reported for all betavoltaic cells with63Ni source. The reported maximum value of Voc for Si-based and forSiC-based betavoltaic cells with 63Ni source is 0.115 V [2] and0.72 V [3], respectively.

Energy conversion efficiency of radiation energy into electrical poweris a key performance parameter of the betavoltaic cell. The conversionefficiency (h) can be defined as the following equation:

h = Pout−max

Psource × hsource

(1)

where Pout – max is the maximum output power of the betavoltaic cell.Psource is the radiation power of the source. hsource is efficiencydefined as the ratio of energy absorbed by the device divided by thetotal radiation energy of the source. Assuming the illuminated area isequal to the area of the cell (2 × 2 mm2), only 0.5 mCi 63Ni-beta effec-tive irradiation is received by the device. Thus, a quarter of Psource isobtained as 32.19 × 107 keV/s. A Monte Carlo simulation solutionwas performed to estimate hsource [10]. According to the simulationresults, the average penetration depth for the 63Ni electron in the GaNtarget is 1520 nm. The deposited energy of injected electrons withtotal energy of 17.4 keV is 5.3 keV in the first 650 nm-depth region (cor-responding to the absorbed layer of the fabricated GaN betavoltaic cellsin this work). So hsource is determined to be 30.5%. With (1), the con-version efficiency (h) is obtained as 2.7 and 0.6% for the GaN betavol-taic cells based on the pin homojunction and the pun homojunctionstructure, respectively. Moreover, if backscatter of the beta particlesfrom the surface of the Ni/Au layers on the top of the device is con-sidered, conversion efficiency h should be larger.

Conclusion: A high open-circuit voltage betavoltaic cell based on aGaN pin homojunction is achieved. Intentional doping with Fe is per-formed in order to compensate the residual donors in the unintentionallydoped GaN layer. Under 0.5 mCi 63Ni source irradiation, the pin homo-junction betavoltaic cell with dimensions of 2 × 2 mm2 exhibits a highopen-circuit voltage Voc of 1.65 V, and the power conversion efficiencyis no less than 2.7%. The results suggest GaN is a highly potential can-didate for the long-life betavoltaic cell.

# The Institution of Engineering and Technology 201119 April 2011doi: 10.1049/el.2011.1143One or more of the Figures in this Letter are available in colour online.

ELECTR

Z.J. Cheng and H.S. San (Pen-Tung Sah Micro-Nano TechnologyResearch Center, Xiamen University, Xiamen 361005, Fujian,People’s Republic of China)

E-mail: sanhs@xmu.edu.cn

Z.H. Feng and B. Liu (Science and Technology on ASIC Laboratory,Hebei Semiconductor Research Institute, Shijiazhuang 050051, Hebei,People’s Republic of China)

X.Y. Chen (Faculty of Science and Engineering, Vestfold UniversityCollege, P.O. Box 2243, Tønsberg N-3103, Norway)

X.Y. Chen: Also with the Pen-Tung Sah Micro-Nano TechnologyResearch Center, Xiamen University, Xiamen, Fujian, China

References

1 Rappaport.: ‘The electron-voltaic effect in p-n junctions induced bybeta-particle bombardment’, Phys. Rev., 1954, 93, pp. 246–247

2 Guo, H., and Lal, A.: ‘Nanopower betavoltaic microbatteries’. Proc. Int.Conf. on Solid State Sensors, Actuators and Microsystems, Boston,MA, USA, June 2003, Vol. 1B3.1, pp. 36–39

3 Chandrashekhar, M.V.S., Thomas, C.I., Li, H., Spencer, M.G., and Lal,A.: ‘Demonstration of a 4H SiC betavoltaic cell’, Appl. Phys. Lett.,2006, 88, p. 033506–1

4 Eiting, C.J., Krishnamoorthy, V., Rodgers, S., and George, T.:‘Demonstration of a radiation resistant, high efficiency SiCbetavoltaic’, Appl. Phys. Lett., 2006, 88, p. 064101–1

5 Qiao, D.Y., Yuan, W.Z., Gao, P., Yao, X.W., Zang, B., Zhang, L., Guo,H., and Zhang, H.J.: ‘Demonstration of a 4H SiC betavoltaic nuclearbattery based on Schottky barrier diode’, Chin. Phys. Lett., 2008, 25,(10), pp. 3798–3800

6 Andreev, V.M.: ‘Nuclear batteries based on III-V semiconductors’, inBower, K.E., Barbanel, Y.A., Shreter, Y.G., Bohnert, G.W. (Eds):‘Polymers, phosphors, and voltaics for radioisotope batteries’ (CRCPress, Boca Raton, FL, 2002), p. 48

7 Look, D.C., and Molnar, P.J.: ‘Degenerate layer at GaN/sapphireinterface: influence on Hall-effect measurements’, Appl. Phys. Lett.,1997, 70, pp. 3377–3379

8 Cheong, M.G., Kim, K.S., Oh, C.S., Namgung, N.W., Yang, G.M.,Lim, K.Y., Suh, E.K., Nahm, K.S., Lee, H.J., Lim, D.H., andYoshikawa, A.: ‘Conductive layer near the GaN/sapphire interfaceand its effect on electron transport in unintentionally doped n-typeGaN epilayers’, Appl. Phys. Lett., 2000, 77, pp. 2557–2559

9 Kozodoy, P., Ibbetson, J.P., Marchand, H., Fini, P.T., Keller, S., Speck,J.S., DenBaars, S.P., and Mishra, U.K.: ‘Electrical characterization ofGaN p-n junctions with and without threading dislocations’, Appl.Phys. Lett., 1998, 73, pp. 975–977

10 Cheng, Z.J., San, H.S., Li, Y.F., and Chen, X.Y.: ‘The designoptimization for GaN-based betavoltaic microbattery’. Proc. Int. Conf.on Nano/Micro Engineered and Molecular Systems, Xiamen, China,January 2010, pp. 575–579

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