CN101975815A - Measuring method of recombination center concentration and trap center concentration in solar-grade crystalline silicon - Google Patents

Abstract

The invention relates to a measuring method of recombination center concentration and trap center concentration in solar-grade crystalline silicon, belonging to the field of measuring method of photovoltaic semiconductor material performance parameters. In the measuring method of the invention, a transient microwave photoconductive minority carrier lifetime tester is used to measure the recombination center concentration and the trap center concentration of a silicon wafer indirectly. The method of the invention is characterized in that the surface of the silicon wafer is passivated, and the transient microwave photoconductive minority carrier lifetime tester is used to measure the minority carrier lifetime of the silicon wafer and the time variation characteristic of the microwave photoconductive transient voltage signal. Because the minority carrier lifetime is resulted by the combined actions of the radiative recombination, the auger recombination and the indirect recombination, when the exciting light is in high injection, the relevance of the minority carrier lifetime and the recombination center concentration is established according to the known injection level and the relevant recombination parameters, thereby obtaining the recombination center concentration of the silicon wafer. The curve of the nonequilibrium carrier concentration with the time decay is obtained by using the relevance of the time variation characteristic of the microwave photoconductive transient voltage signal and the time decay characteristic of the nonequilibrium carrier concentration. By combining the time decay characteristic curve of the nonequilibrium carrier with the trap model expression, the trap center concentration can be obtained by using MATLAB software to perform numerical fitting.

Description

The measuring method of recombination center concentration and trap centre concentration in the solar-grade crystal silicon

Technical field

The present invention relates to utilize the minority carrier life time measuring instrument to measure the method for recombination center concentration and trap centre concentration, belong to the detection technique field of photovoltaic semiconductors material.

Background technology

In a plurality of physical parameters of reflection semiconductor material and device property, minority carrier life time (τ) plays important effect, and measuring minority carrier life time is one of main method of estimating starting material and device technology.On the crystal silicon solar battery production line, adopt RF-MW Photonics to lead decay (MW-PCD) minority carrier lifetime tester and measure the technology commonly used, crucial that minority carrier life time is exactly a detection silicon chip treatment situation.For example, if after silicon chip process chemistry or physical method were handled, minority carrier life time reduced, then infer it to be owing to introduced certain defective in this PROCESS FOR TREATMENT.

Transition metal (for example Fe, Cr and Cu etc.) is a common impurity in the solar-grade pulling of crystals silicon chip, and they generally exist with the form of gap attitude, displacement attitude, complex or precipitation.Studies show that these metallic impurity can influence the efficient of solar cell.In the silicon chip process treatment process, transition metal such as Fe are a metalloid impurity of easily introducing, and their existence may change the concentration of charge carrier in the crystal silicon; May directly introduce deep level center becomes the recombination center, reduces minority carrier lifetime; Also may increase the leakage current of p-n junction, reduce the performance of crystal silicon solar battery.

Yet impurity level may be the recombination center of charge carrier, also may be the trap center.The existence at minority carrier trap center at the few son that is equivalent on the effect capture on a part of recombination center, thereby has reduced the recombination rate of electron-hole pair largely, and this is quite favourable to few sub-device work.There are some researches prove that having of trap center is beneficial to the open-circuit voltage that improves solar cell.Therefore,, but exist, will produce active influence the raising of solar cell performance with the trap center if the Fe impurity of some form is not to exist with the recombination center.

Summary of the invention

The present invention has adopted RF-MW Photonics to lead minority carrier lifetime tester and has measured recombination center concentration and trap centre concentration indirectly, it is characterized in that having following step:

A. be 10 for doping content ¹⁶Cm ^-3The solar-grade silicon chip, carry out surface passivation after, utilize the transient state RF-MW Photonics to lead the minority carrier life time that minority carrier lifetime tester is measured silicon chip;

Injection level when b. adopting the transient state RF-MW Photonics to lead minority carrier lifetime tester measurement minority carrier life time is high injection the (carrier concentration of injection surpasses majority carrier density), and then the expression formula of minority carrier life time can be reduced to In conjunction with

With

Injection level when wherein Δ n is for measurement, σ _n, σ _p, v _Th, C _n, C _pBe known quantity, thereby can be by minority carrier life time value τ _BulkObtain recombination center concentration N _r

C. measure the RF-MW Photonics of silicon chip and lead the transient voltage signal over time, the transient changing relation according to transient voltage signal and nonequilibrium carrier concentration obtains nonequilibrium carrier concentration cutoff characteristic in time;

D. according to the minority carrier life time and the nonequilibrium carrier concentration attenuation characteristic in time of the silicon chip that measures, in conjunction with trap model

With

Numerical fitting obtains the concentration Nt at trap center.

The method of measuring samples recombination center concentration need be carried out at low temperatures as the deep-level transient spectrometer, and positron annihilation, spin paramagnetic resonance or other electrospectrographs need carry out in a vacuum.This measuring method does not need low temperature or vacuum environment, only need carry out surface passivation to sample, the minority carrier life time in the measuring body.Utilize the minority carrier life time that measures under the high injection condition, and, can obtain the recombination center concentration of sample in conjunction with the relation in minority carrier life time and recombination center.Utilize the minority carrier life time value of measuring, carry out numerical fitting, obtain the concentration at trap center in conjunction with trap model.

The principle of measuring recombination center concentration and trap centre concentration among the present invention is as follows:

(1) in semiconductor material, there are multiple mechanisms in three kinds of bodies that influence minority carrier life time: radiation recombination, auger recombination and compound indirectly (SRH), promptly

$\frac{1}{{\mathrm{\τ}}_{\mathrm{bulk}}}=\frac{1}{{\mathrm{\τ}}_{\mathrm{rad}}}+\frac{1}{{\mathrm{\τ}}_{\mathrm{Auger}}}+\frac{1}{{\mathrm{\τ}}_{\mathrm{SRH}}}.---\left(1\right)$

Because when measuring minority carrier life time, the light that is adopted is injected to high the injection, then three kinds of recombination lifetimes can be expressed as respectively

${\mathrm{\τ}}_{\mathrm{Auger},\mathrm{hi}}=\frac{1}{(C_n+C_p)\mathrm{\Δ}{n}^2},---\left(2\right)$

${\mathrm{\τ}}_{\mathrm{rad},\mathrm{hi}}=\frac{1}{\mathrm{B\Δn}},---\left(3\right)$

With ${\mathrm{\τ}}_{\mathrm{SRH},\mathrm{hi}}=(\frac{1}{{\mathrm{\σ}}_p}+\frac{1}{{\mathrm{\σ}}_n})\·\frac{1}{v_{\mathrm{th}}{N}_r}.---\left(4\right)$

In the formula, the horizontal Δ n=4 of the highest injection * 10 ¹⁷Cm ^-3, auger recombination coefficient C _n=2.8 * 10 ^-31Cm ⁶s ^-1, C _p=0.99 * 10 ^-31Cm ⁶s ^-1, radiative recombination coefficient B=9.5 * 10 ^-15Cm ³s ^-1, τ _{SRH, hi}Be the indirect recombination lifetime under the high injection condition of sample, σ _p=2 * 10 ^-15Cm ²And σ _n=3 * 10 ^-14Cm ²Be respectively the capture cross-section of hole and electronics, v _Th=1.1 * 10 ⁷Cm/s is the charge carrier heat movement speed when not distinguishing electronics and hole effective mass, N _rBe defect concentration.So the defective of introducing in the PROCESS FOR TREATMENT exists with the recombination center, can measure by minority carrier life time, determine the concentration in recombination center according to (1)～(4) formula.

(2) according to the Hornbeck-Haynes model, the carrier moving process can be described by the following differential equation in the p type silicon

$\frac{\mathrm{dn}\left(t\right)}{\mathrm{dt}}=G-\frac{n\left(t\right)}{{\mathrm{\τ}}_b}+\frac{n_t\left(t\right)}{{\mathrm{\τ}}_g}-\frac{n\left(t\right)(1-n_t\left(t\right)/N_t)}{{\mathrm{\τ}}_t},---\left(5\right)$

$\frac{{\mathrm{dn}}_t\left(t\right)}{\mathrm{dt}}=-\frac{n_t\left(t\right)}{{\mathrm{\τ}}_g}+\frac{n\left(t\right)(1-n_t\left(t\right)/N_t)}{{\mathrm{\τ}}_t}).---\left(6\right)$

Here n (t) is a nonequilibrium electron concentration in the conduction band, n _t(t) be the electron concentration that trap is captured, G is the right generation rate in light induced electron-hole, τ _bIt is the bluk recombination life-span.The parameter relevant with trap center energy level comprises trap N in equation (5) and (6) _t, electronics is captured on the mean lifetime τ in the trap _gCaptured preceding life-span τ with electronics by the trap center _tEquation (5) has been described not the rate of change of nonequilibrium electron in the conduction band of being captured by trap, and preceding two of equal sign the right is respectively outer the injection and produces and the rate of change of the compound nonequilibrium electron concentration that causes; Three, the 4th is respectively electronics that " escape " comes out from trap and the nonequilibrium electron change rate of concentration that is caused by the electronics that trap is captured.The rate of change of nonequilibrium electron concentration in equation (6) the expression trap.

Sample is led through the non-equilibrium electricity after the illumination, is the coefficient result of charge carrier in nonequilibrium carrier and the trap in the conduction band.Therefore, the carrier concentration n that obtains according to transient state PCD _aCan be expressed as

$n_a\left(t\right)=n\left(t\right)+\frac{{\mathrm{\μ}}_p}{{\mathrm{\μ}}_p+{\mathrm{\μ}}_n}{n}_t\left(t\right).---\left(7\right)$

Wherein, μ _nAnd μ _pBe respectively the mobility in electronics and hole.

Because recombination center and trap center are that defect and impurity causes, therefore, the concentration at recombination center and trap center ratio in the high material of defect and impurity concentration is higher in the low material of defect density.

The inventive method is different from the method for defective in the common measuring samples, and the one, need under vacuum condition, not carry out because of this method, the 2nd, because this method is suitable for large-sized sample.And characteristics of the present invention to be samples only need carry out surface passivation preferably, just can utilize minority carrier life time promptly to measure recombination center concentration and trap centre concentration indirectly.

Description of drawings

Relation between Fig. 1 recombination center concentration and the minority carrier life time, σ _p, σ _nBe respectively the capture cross-section in hole, hole, v _ThBe charge carrier heat movement speed, N _rBe defect concentration; B is a radiative recombination coefficient, C _n, C _pBe respectively the auger recombination coefficient in electronics and hole, Δ n is the injection level, and aforementioned parameters is known quantity.τ _{SRH, hi}, τ _{Rad, hi}, τ _{Aug, hi}Be respectively indirect recombination lifetime, radiative recombination lifetime and auger recombination life-span under the high injection condition of sample, τ _BulkBody life time for sample after the passivation.

The recombination center concentration that Fig. 2 obtains in conjunction with the relation (shown in Figure 1) of the minority carrier life time (shown in Fig. 2 (a)) that measures sample and recombination center concentration distribute (shown in Fig. 2 (b)).(a) minority carrier life time; (b) recombination center concentration

The MW-PCD transient voltage of Fig. 3 sample and nonequilibrium carrier concentration attenuation characteristic thereof.(a) MW-PCD transient voltage; (b) nonequilibrium carrier concentration attenuation characteristic.

The measurement of Fig. 4 minority carrier life time combines the graph of a relation that obtains trap with trap model.G is an electron production rate, n (t), n _t(t) electron concentration that is respectively in the conduction band and is captured by trap; τ _t, τ _gBe respectively that electronics is captured by trap and the time of the trap of escaping out; μ _n, μ _pBe respectively the mobility in electronics and hole; E _CBe conduction level, N _CBe the conduction band effective density of states.

Embodiment

After now embodiments of the invention specifically being described in.

Embodiment 1

Concrete steps in the present embodiment are as follows:

(1) utilizes plasma to strengthen gas phase deposition technology and on sample, deposit SiN _x: the H film, utilize the minority carrier life time of transient state MW-PCD minority carrier lifetime tester measuring samples then, the result is shown in Fig. 2 (a).

(2) utilization graph of a relation as shown in Figure 1 in conjunction with the minority carrier life time and equation (1)～(4) of sample, obtains the recombination center concentration of sample, and the result is shown in Fig. 2 (b).

(3) the transient voltage attenuation characteristic (as Fig. 3 (a)) in time of utilizing the MW-PCD minority carrier lifetime tester to measure obtains nonequilibrium carrier concentration cutoff characteristic (as Fig. 3 (b)) in time.

(4) carry out match in conjunction with Fig. 3 (b) with the Hornbeck-Haynes model and obtain trap.Utilize MATLAB software, nonequilibrium carrier die-away curve shown in Fig. 3 (b) and equation (5)～(7) are carried out match.Stop illumination constantly at t=0, from begin photo-generated carrier generation rate G=0 this moment, nonequilibrium carrier concentration begins decay.Because the rate of decay of carrier concentration is fast than the rate of decay of carrier concentration in the trap in the conduction band, so can think that all this moment traps are filled (n _t(0)=N _t), and the carrier concentration (n that captures far above trap of the nonequilibrium carrier concentration in the conduction band (n (t)) _t(t)).The level of energy E that known FeB is right _tBe positioned at 0.29eV (E under the conduction band _cOne 0.29eV) locates, utilize formula

$E_c-E_t=\mathrm{kT}\ln \left(\frac{N_c{\mathrm{\τ}}_g}{N_t{\mathrm{\τ}}_t}\right)---\left(8\right)$

Can obtain the escapement ratio τ to causing by FeB _g/ τ _tWith trap N _tRelation be respectively τ _g/ (N _tτ _t)=2.45 * 10 ^-15Cm ^-3, the effective density of states N of conduction band here _cBe 2.86 * 10 ¹⁹Cm ^-3Adjust initial nonequilibrium carrier concentration n (0), trap N in the conduction band _t, trap constant τ _tAnd τ _g, experimental data is carried out match, can obtain the trap (as shown in Figure 4) of sample.The measurement result of present embodiment sees the following form 1:

The different Fe-B of table 1 sample are to the recombination center concentration and the trap centre concentration of concentration position correspondence

Annotate: with Fig. 2 is example, and the center of two-dimensional distribution is that true origin (0,0) is set up the two-dimensional coordinate axle

By table as seen, utilize the minority carrier life time of minority carrier lifetime tester measuring samples, utilize the relation in minority carrier life time and recombination center can obtain recombination center concentration then.In conjunction with minority carrier life time measuring-signal and trap model, can obtain the trap centre concentration.

Certainly, the accurate measurement of recombination center concentration and trap centre concentration is measured as prerequisite with minority carrier life time accurately.Therefore, to the passivation of silicon chip surface excellence, to reduce surface recombination be accurately to measure the condition precedent of recombination center concentration and trap centre concentration.

Measuring method of the present invention can realize recombination center concentration and the measurement of trap centre concentration in the semiconductor material.Measuring method of the present invention is simple, does not need expensive low temperature or vacuum equipment, has positive effect for recombination center concentration and the trap centre concentration measured in the semiconductor material on laboratory or industrial production line.

Claims (1)

1. the measuring method of recombination center concentration and trap centre concentration in the solar-grade crystal silicon is characterized in that having following steps:

A. be 10 for doping content ¹⁶Cm ^-3The solar-grade silicon chip, carry out surface passivation after, utilize the transient state RF-MW Photonics to lead the minority carrier life time that minority carrier lifetime tester is measured silicon chip;

Injection level when b. adopting the transient state RF-MW Photonics to lead minority carrier lifetime tester measurement minority carrier life time is injected for high, and then the expression formula of minority carrier life time can be reduced to

In conjunction with

With

Injection level when wherein Δ n is for measurement, σ _n, σ _p, v _Th, C _n, C _pBe known quantity, thereby can be by minority carrier life time value τ _BulkObtain recombination center concentration Nr;

C. measure the RF-MW Photonics of silicon chip and lead the transient voltage signal over time, the transient changing relation according to transient voltage signal and nonequilibrium carrier concentration obtains nonequilibrium carrier concentration cutoff characteristic in time;

D. according to the minority carrier life time and the nonequilibrium carrier concentration attenuation characteristic in time of the silicon chip that measures, in conjunction with trap model

With Numerical fitting obtains the concentration Nt at trap center.

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