GaAs diluted magnetic diode

Current Applied Physics 13 (2013) 537e543

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Photovoltaic characterization of n-CdTe/p-CdMnTe/GaAs diluted magnetic diode I.S. Yahia a, b, *, F. Yakuphanoglu c, S. Chusnutdinow d, T. Wojtowicz d, G. Karczewski d a

Nano-Science & Semiconductor Labs., Physics Department, Faculty of Education, Ain Shams University, Roxy, Cairo, Egypt Department of Physics, Faculty of Science, King Khalid University, P.O. Box 9004, Abha, Saudi Arabia c , Turkey Department of Physics, Faculty of Science, Firat University, Elazig d Institute of Physics, Polish Academy of Sciences, Al. Lotnikow, 32/46, 02-668 Warszawa, Poland b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 May 2012 Received in revised form 21 August 2012 Accepted 26 September 2012 Available online 9 October 2012

A CdTe/CdMnTe heterojunction magnetic diode for photovoltaic applications was fabricated by using molecular beam epitaxy (MBE). The ideality factor and the potential barrier height of the diode were determined to be 1.25 and 0.836 eV, respectively. Photovoltaic parameters of the studied device were determined at various illumination intensities. The highest open circuit voltage of the CdTe/CdMnTe heterostructure was equal to 0.56 V at the illumination intensity of 130 mW/cm2. The reverse current of the n-CdTe/p-CdMnTe/GaAs diode increases with the increasing illumination intensities. The obtained results suggest that n-CdTe/p-CdMnTe/GaAs diode can be used as a photodiode in photovoltaic and photodetector applications. Ó 2012 Elsevier B.V. All rights reserved.

Keywords: CdTe/CdMnTe Photovoltaic effect Currentevoltage characterization under dark and illuminations Photovoltaic parameters

1. Introduction IIeVI semiconductors offer a lot of opportunities for investigations of various physical properties. This is due to the large span of energy gaps characterizing them as well as the facility of the growth of IIeVI ternary alloys over a significant composition range [1]. Replacement of group II element by 3d transition metal ion makes it semi-magnetic and gives rise to interesting electrical, magnetic, optical, elastic, phonon properties and the pressureinduced phase transition [1e5]. Manganese-doped IIeVI and IIIeV as diluted magnetic semiconductors (DMS) have recently attracted a great attention as a new class of semiconductor materials, as they exhibit an interesting combination of magnetism and electrical properties, which are essential for future generation spintronics device applications and a promising materials photovoltaic conversion [6,7]. Cadmium manganese telluride (CdMnTe) is a semiconductor material with wide range of applications in IR detectors, magnetooptical isolators, solar cells, visible and near-IR lasers and possesses a bandgap suitable for optical applications [8]. Some of the well

* Corresponding author. Present address: Nano-Science & Semiconductor Labs., Department of Physics, Faculty of Education, Ain Shams University, Roxy, Cairo, Egypt. Tel.: þ966 549224884. E-mail addresses: [email protected], [email protected], isyahia@ gmail.com (I.S. Yahia). 1567-1739/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cap.2012.09.018

known heavy atom semiconductor detectors of gamma radiation, operating at room temperature, including CdTe and CdZnTe were investigated earlier by many researchers [9e11]. Today, the MBE technique is used for the epitaxial growth of a wide range of different materials and heterostructures with interesting physical properties and applications in modern devices. However, when the MBE growth is used for fabrication of heterostructures consisting of different material systems, one has to consider that constituent elements of one material might be impurities for another. In order to achieve the highest possible quality of both material systems, the respective layers are grown in different growth chambers [12]. The previous work by I.S. Yahia et al. [13,14] showed that the employing of MBE technology for photovoltaic applications. P-ZnTe/n-CdMnTe/n-GaAs [13] and ZnTe/CdTe/GaAs [14] heterojunction diodes were grown by MBE machine. The currentevoltage characteristics of the studied device showed the photovoltaic properties under different illuminations. Photovoltaic parameters such as the short-circuit current Isc and open-circuit voltage Voc, power P ¼ IV, maximum power Pmax, maximum current IM and maximum voltage VM were calculated at different light intensities. The photosensitivity and the responsivity of the prepared device were calculated at different illumination intensities and the prepared device showed a high sensitivity to the light [12,13]. In present work we investigate a CdTe/CdMnTe heterostructure diode that was prepared for photovoltaic conversion by using

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molecular beam epitaxy (MBE) technology. The light-currente voltage curves under different illuminations were studied and the related photovoltaic parameters were deduced. The illumination dependence of open-circuit voltage (Voc) and short-current (Isc) were studied in details. The photosensitivity and responsivity of the photovoltaic device were calculated.

The investigated heterostructure e n-CdTe/p-CdMnTe/GaAs e was grown by using MBE technique in the EPI-620 MBE system at the Institute of Physics, Polish Academy of Sciences, in Warsaw, Poland. Ultra-pure elements (Cd, Mn, Te) were deposited in ultrahigh vacuum to form the heterojunction. The p-type layers were doped by nitrogen from the radio frequency (RF) plasma source using ultra-pure N2 gas, whereas ZnI2 (5N) was used as a solid source of iodine donors for the n-type doping. Firstly, a 300 nm thick CdTe layer was evaporated on pþ-GaAs (100) epi-ready substrate. Then a 1.2 um thick CdMnTe absorber with graded manganese content was formed on the top of the initial CdTe layer. The both layers were undoped, thus slightly p-type. The manganese concentration in the absorber was gradually changed by changing the temperature of the Mn cell from zero (pure CdTe) to about 3% at the top by changing the temperature of the Mn source effusion cell from 740 to 908  C. In result, the energy gap of the absorber was changed from 1.5 eV (the energy gap of CdTe) up to about 2.23 eV (the gap of Cd0.60Mn0.40Te). The structure was capped by a 300 nm thick CdTe layer highly n-type doped by iodine donors from a ZnI2 cell at temperature 195  C. The top contact to n-type CdTe layer was formed by soldering an indium point contact (mesh contact) and annealing for 3 min at 200  C. Evaporated and annealed Indium was also used for forming the ohmic back contact to the pþ-GaAs. The total area of the device was A ¼ 0.35  0.45 cm2. Currentevoltage (IeV) measurements were performed using a computer controlled Keithley 4200-SCS semiconductor characterization system. Two terminal cables of this device with two Source Meter Unit (SMU1 and SMU2) were connected to the specially designed homemade holder for a point contact made from brass. The tungstenehalogen lamp was calibrated at the references point (100 mW/cm2) i.e. AM1.5 according to (Solar Cell Tester M 54A) inside the Arab International Optronics Co. at Cairo, Egypt and Small-Area ClassBBA Solar Simulator at Firat University, Elazig, Turkey. The intensity of light was varied by changing the voltage across the tungsten lamp and light intensity was measured by solar power meter (TM-206). The distance between the lamp and the sample is approximately equals 30 cm to avoid the heating flow from the lamp to the device.

-3

2.0x10

-3

1.5x10

I, (A)

2. Experimental

-3

2.5x10

-3

1.0x10

-4

5.0x10

0.0 -2

-1

0

1

2

V, (Volt) Fig. 1. The linear currentevoltage characteristics of n-CdTe/p-CdMnTe/GaAs magnetic diode at room temperature.


qV I ¼ I0 exp nkB T

(1)

where q is the electronic charge, V is the forward biasing voltage, n is the diode ideality factor, kB is the Boltzmann’s constant, T is the absolute temperature and I0 is the reverse saturation current. The saturation current according to thermionic emission mechanism can be written as [15]:


qfb ; I0 ¼ AA* T 2 exp kB T

(2)

where A is the diode contact area, A* is the effective Richardson constant and equals 74.4 A/(cm2 K2) for p-type GaAs [20], 4b is the effective barrier height at zero biasing. The saturation current I0 was calculated by extrapolating the linear line to a zero applied biasing voltage at room temperature and its value equals 4.5 nA. The ideality factor is calculated from the slope of the linear region of the forward bias ln(I)eV plot as shown in Fig. 2 and can be written from Eq. (1) as [15,21]:


n ¼

q kB T




 dV ; dðln IÞ

(3)

-16.0 -16.5

3. Results and discussion -17.0

3.1. Dark currentevoltage characteristics

ln(I)

-17.5

The current voltage (IeV) characteristics are the most important measurements to study the electrical behavior of any diode. The IeV measurements have been used to extract the diode (junction) parameters such as the ideality factor, the barrier height, the series resistance, and the interface states [15e19]. Current voltage characteristics of n-CdTe/p-CdMnTe/GaAs magnetic diode are plotted in linear scale in Fig. 1. The rectification ratio (RR) is defined as the ratio of the forward current to the reverse current at a certain value of the applied voltage. The rectification ratio at the biasing voltage of 2 V and at room temperature equals 539 [13,20]. Based on the thermionic emission model, the current flow the studied diode at lover biasing voltage can be given as [15]:

-18.0 -18.5 -19.0 -19.5 0.00

0.02

0.04

0.06

0.08

0.10

V, (Volt) Fig. 2. Ploting of ln(I)eV for n-CdTe/p-CdMnTe/GaAs magnetic diode at room temperature.

I.S. Yahia et al. / Current Applied Physics 13 (2013) 537e543

where dV/d(ln I) is the slope of linear plot of ln(I)eV. The value of diode ideality factor equals 1.25. For an ideal diode n equals one. Usually, n has a value greater than unity. High values of n can be attributed to the presence of native oxide layer or/and barrier inhomogeneities. The higher value of the ideality factor has been attributed to a particular distribution of the interface states [22], the image-force effect, recombination-generation; and tunneling may be other possible mechanisms that could lead to an ideality factor value greater than unity [15,23e27]. 4b is the zero-bias barrier height (BH), which can be obtained from the following equation [15,21,22]:

fb ¼




!  kB T AA* ln ; q I0

I ¼

eAmNv 330 d2lþ1 eP0 kB Tt

l

Table 1 The calculated parameters for SCLC regions for the studied device. Tt

l

m

Region

293 1465 586

1 5 2

2 6 3

II III VI

voltages, the slope of the plot tends to decrease (¼3) because the device approaches the ‘trap-filled’ limit when the injection level is high whose dependence is the same as in the trap-free SCLC [31]. 3.2. Currentevoltage characteristics under illumination

(4)

The calculated value of the 4b depends on the reverse saturation current, I0. The calculated value 4b at room temperature equals 0.836 eV. To study the conduction mechanism for the studied device in the forward biasing voltage, we plot the double logarithmic of log (I)elog (V) as shown in Fig. 3. It is clear from Fig. 3 that the forward biasing characteristics of the studied diode shows three distinct linear regions, indicating different conduction mechanisms. As can be seen from the double logarithmic forward biasing IeV plot in Fig. 3, the charge transport mechanism is mainly governed by space charge limited current (SCLC) process. The log (I)elog V plot supports the power law behavior of the current IfV m with different exponential values (m) [13]. The m values indicate that at the higher voltage region, the carrier transport may be dominated by an SCLC [13,27], where current increases superlinearly, i.e., IfV m2 suggesting that the traps are exponentially distributed. This confirms an SCLC model controlled by an exponential distribution of traps for the studied device. Therefore, the current passes through our sample can be expressed as [28]:




539

Currentevoltage (IeV) measurements are the standard method for evaluating the electrical performance of solar cells. The device under test is placed under a calibrated light for different light intensity. The working principle of a solar cell is the producing of a photocurrent under illumination, even at zero bias (short circuit condition). The photocurrent at zero bias is called the short-circuit current (Isc). It is noticed that the magnitude of the short circuit current is proportional to the illuminated area of the studied device [32e34]. In the dark condition, most of solar cells exhibit rectifying diode properties [32e34] i.e. they admit a much larger current under the forward voltage biasing than its reversed voltage biasing. The dark current flows in the device as a function of an applied forward voltage biasing, acts in the opposite direction of the photocurrent. The overall current in the device under illumination and at an applied forward bias is thus the short-circuit current reduced by the opposing dark current. A reasonable approximation for the most solar cells is that the currentevoltage response can be taken as the sum of the short-circuit current and the dark current as follows [32e34]:

JðVÞ ¼ Jsc  Jdark ; V lþ1

(5)

where m is the mobility of carrier charges, Nv is the effective density of states in the valance band edge, 3 is the dielectric constant of the semiconductor, P0 is the trap density per unit energy range at the valence band [29], and l is a parameter given by l ¼ m  1 ¼ Tt =T. The value Tt is a characteristic temperature of the exponential distribution of the traps [30]. The values of m, l and Tt at different temperatures were calculated and are given in Table 1. At the higher

(6)

where Jsc is the current density under illumination condition and Jdark is the current density under dark condition. At a certain voltage, the dark current cancels out the short-circuit current and so, the net current is zero. This voltage is known as the open-circuit voltage Voc. The operating range of the solar cells then stretches from 0 bias to Voc [32e34]. The light-currentevoltage curves of the studied device are shown in Fig. 4 under forward and reverse 3.0m

D a rk 0 .2 m W /c m

2.5m

(III)

I

2.0m

3

V

log(I)

-5

I

I, (A)

1.5m

(II)

2 0 m W /c m 3 0 m W /c m 4 0 m W /c m

-4 6

V

5 0 m W /c m 6 0 m W /c m 7 0 m W /c m

1.0m

8 0 m W /c m 9 0 m W /c m 1 0 0 m W /c m

-6

500.0µ

1 1 0 m W /c m 1 2 0 m W /c m 1 3 0 m W /c m

-7

-1.5

5 .0 m W /c m 1 0 m W /c m

-3

-8

2 .5 m W /c m

I

(I)

0.0

2

V

-500.0µ

-1.0

-0.5

0.0

0.5

log(V) Fig. 3. The forward biasing plotting of log(I) versus log(V) for n-CdTe/p-CdMnTe/GaAs magnetic diode at room temperature.

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

V, (V) Fig. 4. Light-currentevoltage of the studied solar cell device under different illuminations for n-CdTe/p-CdMnTe/GaAs magnetic diode solar cell device.

I.S. Yahia et al. / Current Applied Physics 13 (2013) 537e543

Isc ¼ Iph ¼ Lg ;

(7)

where ISC ¼ Iph is the generated photocurrent under different illuminations, L is the light intensity and g is an exponent, which depends on the recombination mechanism. Plot of ln Isc  ln L is shown in Fig. 7. This plot yields a linear relation and from its slope, value of the exponent g was determined and equal 1.14. The g values for 0.5 and 1.0 and larger than 1 correspond to bimolecular recombination and mono-molecular recombination and supralinear recombination (strong recombination at the surface), respectively [39,40]. Whereas, the value of the exponent lies

0.2 mW/cm 2.5 mW/cm 5 mW/cm 10 mW/cm 20 mW/cm 30 mW/cm 40 mW/cm 50 mW/cm 60 mW/cm 70 mW/cm 80 mW/cm 90 mW/cm 100 mW/cm 110 mW/cm 120 mW/cm 130 mW/cm

200.0µ

I, (A)

150.0µ

100.0µ

50.0µ

250.0µ

0.6

200.0µ

0.5 0.4

150.0µ

0.3 100.0µ

Isc, (A)

Voc, (V)

voltage biasing. When the solar device is illuminated, the IV curve is shifted down by the amount of photocurrent generated Iph ¼ Isc . The shifting of IeV curves in fourth quadrant representing that the cell is a generator of electricity. From Fig. 3, the fourth quadrant part is extracted and shown in Fig. 5 for more illustrations. From Fig. 5, the solar cell parameters were extracted and interpreted throughout the applied theory in this field. It is clear that the I-V curves in the reverse voltage biasing is influenced much under the effect of different illumination intensities suggesting that n-CdTe/ p-CdMnTe/GaAs magnetic diode is a good candidate for photovoltaic and photodetector applications. The open-circuit voltage, Voc , is the maximum photovoltage that can be generated in the cell and corresponds to the voltage where current under illumination is zero. The maximum current that can run through the cell at zero applied voltage is called the shortcircuit current, Isc . The short-circuit current (Isc) and the open circuit voltage (Voc) are measured as a function of light intensity L for n-CdTe/p-CdMnTe/GaAs magnetic diode solar cell is shown in Fig. 6. It is clear that the open-circuit voltage depends on the applied illumination L as Voc z ln L [35] with an exponential increase with the applied illuminations. Also, the studied device shows high value of open circuit voltage of 0.335 V at lower intensity (0.2 mW/cm2) followed by an increase in its values to 0.56 V at higher intensity (130 mW/cm2). The short circuit current increases with increase the illumination intensities suggested the linearity of its behavior at a wide range of illuminations [13]. Short-circuit current, Isc, shows a linear variation with light illumination intensities which is in good agreement with the theory which can be explained by the relation [36e38]:

0.2 0.1

V

50.0µ

I

0.0

0.0 0

20

40

60

80

100

120

2

L, (mW/cm ) Fig. 6. Variation of Isc and Voc against light intensity L for n-CdTe/p-CdMnTe/GaAs magnetic diode solar cell device.

between 0.5 and 1.0 for continuous distribution of trapping centers [41,42]. The operating regime of the solar cell is the range of bias, from 0 to Voc, in which the cell delivers power. The power delivered from a solar cell at a certain potential equals the product of the current and voltage [43]:

P ¼ I  V;

(8)

Fig. 8 represents the cell power as a function of the potential between 0 and VOC. The power P reaches a maximum at the cell’s operating point or maximum power point, Pmax . The maximum electrical power Pmax equals the product of the maximum current and the maximum voltage values (Pmax ¼ Imax Vmax ). The extracted power parameters such as: Pmax , Vmax , Imax are tabulated in Table 2. It is clear that Pmax increases with increasing the incident light. The open-circuit voltage (Voc) and short-circuit current (Isc) are strongly dependent on the series resistance (Rs) as well as on the diode ideality factor (n) as follows [44e47]:

  
qðV  IRs  1  I; Isc ¼ Iph ¼ Io exp nkT

(9)

-8

-9

-10

lnIsc

540

-11

-12

-13

0.0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

V, (V)

1

2

3

4

5

lnL Fig. 5. The fourth quadrant light-currentevoltage of the studied solar cell device under different illuminations for n-CdTe/p-CdMnTe/GaAs magnetic diode solar cell device.

Fig. 7. Illumination dependence of the short-circuit current Isc n-CdTe/p-CdMnTe/GaAs magnetic diode solar cell device.

I.S. Yahia et al. / Current Applied Physics 13 (2013) 537e543

60.0µ

0 .2 m W /c m

541

0.60

2 .5 m W /c m 5 m W /c m

50.0µ

1 0 m W /c m

0.55

2 0 m W /c m 4 0 m W /c m 5 0 m W /c m

P (W)

6 0 m W /c m 7 0 m W /c m

30.0µ

Voc, (V)

3 0 m W /c m

40.0µ

0.50 0.45

8 0 m W /c m 9 0 m W /c m

0.40

1 0 0 m W /c m

20.0µ

1 1 0 m W /c m 1 2 0 m W /c m

0.35

1 3 0 m W /c m

10.0µ 0

50µ

100µ

0.0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

V (V) Fig. 8. Voltage dependence of the delivered power P for n-CdTe/p-CdMnTe/GaAs magnetic diode solar cell device.

Voc ¼


 nkT Isc ln þ1 ; q I0

(10)

where I is the total output current and I0 is the diode saturation current, Rs is the series resistance, n is the diode ideality factor, k is the Boltzmann’s constant and q is the electric charge. Series resistance reduces the short circuit current and hence, the open circuit voltage also. Plotting of open circuit voltage Voc versus short circuit current Isc is shown in Fig. 9. It is observed that Voc increases exponentially with increasing of Isc satisfying Eq. (6) and the fitted curve according to this equation is shown on the experimental points. According to Eq. (6), VOC is proportional to the log Isc, and it increases with the increasing of light intensity [47]. The variation of open circuit voltage Voc versus against the illumination intensity is satisfying the following equation [48,49]:

Voc ¼

150µ

200µ

Isc, (A)

nkT ln L þ Const:; e

(11)

Fig. 10 shows the variation of Voc with ln L which yield a straight line satisfying the above equation. From this plot, the junction ideality factor n under illumination is calculated from the slope of this straight line and its value equal ¼ 1.978 [49]. This value is close

Fig. 9. Plotting of open circuit voltage Voc versus short circuit current Isc for n-CdTe/pCdMnTe/GaAs magnetic diode solar cell device.

to the value deduced for this device under dark; the difference may be due to illumination effect. The Photosensitivity (PS) is defined as the ratio of photoconductivity to the dark conductivity as follows [50e52]:

PS ¼

R ¼

Iph ¼ Isc Photo  current ¼ ; Incident  power Pincident

IM (A)

Pmax (W)

0.2 2.5 5 10 20 30 40 50 60 70 80 90 100 110 120 130

0.225 0.225 0.295 0.325 0.36 0.36 0.36 0.36 0.35 0.335 0.335 0.32 0.33 0.335 0.33 0.355

1.44E6 1.23E6 3.16E6 5.18E6 1.31E5 2.22E5 2.99E5 3.99E5 5.05E5 6.44E5 7.52E5 8.99E5 1.02E4 1.10E4 1.35E4 1.60E4

3.25E7 2.77E7 9.34E7 1.69E6 4.70E6 7.99E6 1.08E5 1.44E5 1.77E5 2.16E5 2.52E5 2.88E5 3.37E5 3.67E5 4.46E5 5.67E5

(13)

0.60 0.55

Voc, (V)

VM (V)

(12)

The variation of the photosensitivity versus the illumination intensities for the investigated device is shown in Fig. 11 at different dark current. It is clear that the studied device shows a high photosensitivity increase with increasing illumination intensities. The enhancement of the photoconductive sensitivity is due to the electronehole pairs excited by the incident light. Also, the high quality of device preparation support its high photoconductivity allowed its application in optoelectronic devices. The responsivity (R) can be defined as the sensitivity of the solar cell device to the effect light, in other words, it is a measure of the effectiveness of the conversion of the light power into the electrical current and it is defined as [33,53]:

Table 2 The extracted solar power parameters for n-CdTe/p-CdMnTe/GaAs magnetic diode solar cell device. L (mW/cm2)

Photo  current ; Dark  current

0.50 0.45 0.40 0.35 1

2

3

4

5

2

lnL, (L in mW/cm ) Fig. 10. The variation of Voc versus the illumination intensity ln L for n-CdTe/pCdMnTe/GaAs magnetic diode solar cell device.

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I.S. Yahia et al. / Current Applied Physics 13 (2013) 537e543

2500

Acknowledgments V= 1 V V = 1 .5 V V= 2 V

0.8

Research was partially supported by the European Union within European Regional Development Fund, through grant Innovative Economy (POIG.01.01.02-00-108/09).

0.6

2000

Photosensitivity

0.4

1500

0.2 0.0

1000

References 0

20

40

60

80

100

120

V = 0 .1 V , I= 9 .6 1 E -0 8 A v = 0 .5 V , I= 8 .5 0 E -0 6 A

500

V = 1 V , I= 2 .6 7 E -0 4 A V = 1 .5 V I= 1 .0 1 E -0 3 A V = 2 V I= 2 .2 1 E -0 3 A

0 0

20

40

60

80

100

120

2

L, (mW/cm ) Fig. 11. Plotting of the photosensitivity versus the illumination intensities for the investigated device at different dark current.

1.8µ

Responsivity

1.6µ

1.4µ

1.2µ

1.0µ

800.0n 0

20

40

60

80

100

120

2

L, (mW/cm ) Fig. 12. Plotting of the responsivity versus the illumination intensities for the investigated device.

where Iph ¼ Isc (A) is the photocurrent under different illumination intensities through the absorption of incident light and Pincident is the measured incident optical power in (mW/cm2). Plotting of responsivity versus the incident power light is shown in Fig. 12. It is clear that the responsivity is increased with increasing the incident illumination suggesting the possibility of the power delivered from this device. 4. Conclusions The electrical and photovoltaic properties of CdTe/CdMnTe heterojunction diode fabricated by MBE technology have been investigated. The electrical parameters such as ideality and barrier height value of the diode were determined using currentevoltage characteristics. The diode exhibits a photovoltaic behavior with a short circuit current and open circuit voltage. QE efficiency spectra suggest that the most absorption of light is near IR region according to the band gap of the studied device. The reserve current evoltage characteristics of the diode under various illuminations intensities indicate that the diode behaves as a photodiode. The obtained results suggest that n-CdTe/p-CdMnTe/GaAs diode can be used as a photodiode in photosensing applications.

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